PACKING BOX CAPABLE OF ADJUSTING ITS SIZE FIELD OF THE INVENTION The present invention relates to a packing box which is 5 assembled in various sizes based on using requirements. BACKGROUND OF THE INVENTION A conventional packing box is adapted to package a product in a fixed size, so as packing another product of a different 10 size, another packing box corresponding to another product of the different size is produced. The present invention has arisen to mitigate and/or obviate the afore-described disadvantages. 15 SUMMARY OF THE INVENTION The primary objective of the present invention is to provide a packing box which is assembled in various sizes based on using requirements. To obtain above-mentioned objective, a packing box 20 provided by the present invention contains: an accommodating casing, a first side casing, a second side casing, plural bottom casings of various sizes, plural top casings of various sizes, and a plurality of binding straps. The accommodating casing is formed in a U shape and 25 includes a plurality of first spaced holes defined on two upper ends 1 of two side surfaces thereof and a plurality of second spaced holes defined on two lower ends of the two side surfaces thereof. The first side casing is formed in a U shape and is fitted onto a first end of the accommodating casing, and the first side 5 casing includes plural first spaced orifices formed on two upper ends of two side surfaces thereof and plural second spaced orifices formed on two lower ends of the two side surfaces thereof, and when the first side casing moves away from or close to the accommodating casing, the plural first spaced orifices align with 10 some of the plurality of first spaced holes, and the plural second spaced orifices align with some of the plurality of second spaced holes. The second side casing is formed in a U shape and is fitted onto a second end of the accommodating casing, and the second 15 side casing includes plural first spaced apertures arranged on two upper ends of two side surfaces thereof and plural second spaced apertures arranged on two lower ends of the two side surfaces thereof, and when the second side casing moves away from or close to the accommodating casing, the plural first spaced 20 apertures align with another of the plurality of first spaced holes, and the plural second spaced apertures align with another of the plurality of second spaced holes. Each bottom casing covers a bottom of each of the accommodating casing, the first side casing, and the second side 25 casing, wherein a size of the each bottom casing corresponds to a 2 length of a combination of bottoms of the accommodating casing, the first side casing, and the second side casing. Each top casing covers a top of each of the accommodating casing, the first side casing, and the second side casing, wherein a 5 size of the each top casing corresponds to a length of a combination of tops of the accommodating casing, the first side casing, and the second side casing. Each binding strap inserts into the packing box from each first spaced orifice or each first spaced aperture via each first 10 spaced hole, and the each binding strap extends out of a packing box from each second spaced hole through each second spaced orifice or each second spaced aperture, thereafter the each binding strap surrounds the each bottom casing and inserts into the packing box from the each second spaced orifice or the each second spaced 15 aperture via the each second spaced hole, and the each binding strap extends outwardly from the each first spaced hole through the each first spaced orifice or each first spaced aperture so as to surround the each top casing. The each biding strap includes a fastener, a male fastening 20 face formed on a top of a distal end thereof, and a female fastening face arranged on a bottom of a distal end thereof, wherein the fastener fastens with the male fastening face and the female fastening face so that the each binding strap fixes the packing box. 25 BRIEF DESCRIPTION OF THE DRAWINGS 3 FIG. 1 is a perspective view showing the assembly of a packing box according to a first embodiment of the present invention. FIG. 2 is a perspective view showing the exploded 5 components of the packing box according to the first embodiment of the present invention. FIG. 3 is a perspective view showing the assembly of a packing box according to a second embodiment of the present invention. 10 FIG. 4 is a perspective view showing the assembly of a packing box according to a third embodiment of the present invention. FIG. 5 is a perspective view showing the exploded components of a part of the packing box according to the first 15 embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to FIGS. 1, 2 and 5, a packing box 1 20 according to a first embodiment of the present invention comprises: an accommodating casing 10, a first side casing 20, a second side casing 30, plural bottom casings 40 of various sizes, plural top casings 50 of various sizes, and a plurality of binding straps 60. The accommodating casing 10 is formed in a U shape and 25 includes a plurality of first spaced holes 11 defined on two upper 4 ends of two side surfaces thereof and a plurality of second spaced holes 12 defined on two lower ends of the two side surfaces thereof. The first side casing 20 is formed in a U shape and is fitted onto a first end of the accommodating casing 10, wherein the first 5 side casing 20 includes plural first spaced orifices 21 formed on two upper ends of two side surfaces thereof and plural second spaced orifices 22 formed on two lower ends of the two side surfaces thereof, and when the first side casing 20 moves away from or close to the accommodating casing 10, the plural first spaced orifices 21 10 align with some of the plurality of first spaced holes 11, and the plural second spaced orifices 22 align with some of the plurality of second spaced holes 12. The second side casing 30 is formed in a U shape and is fitted onto a second end of the accommodating casing 10, wherein 15 the second side casing 30 includes plural first spaced apertures 31 arranged on two upper ends of two side surfaces thereof and plural second spaced apertures 32 arranged on two lower ends of the two side surfaces thereof, and when the second side casing 30 moves away from or close to the accommodating casing 10, the plural first 20 spaced apertures 31 align with another of the plurality of first spaced holes 11, and the plural second spaced apertures 32 align with another of the plurality of second spaced holes 12. Each bottom casing 40 covers a bottom of each of the accommodating casing 10, the first side casing 20, and the second 25 side casing 30, wherein a size of the each bottom casing 40 5 corresponds to a length of a combination of bottoms of the accommodating casing 10, the first side casing 20, and the second side casing 30. Each top casing 50 covers a top of each of the 5 accommodating casing 10, the first side casing 20, and the second side casing 30, wherein a size of the each top casing 50 corresponds to a length of a combination of tops of the accommodating casing 10, the first side casing 20, and the second side casing 30. Each binding strap 60 inserts into the packing box 1 from 10 each first spaced orifice 21 or each first spaced aperture 31 via each first spaced hole 11, and the each binding strap 60 extends out of the packing box 1 from each second spaced hole 12 through each second spaced orifice 22 or each second spaced aperture 33, thereafter the each binding strap 60 surrounds the each bottom 15 casing 40 and inserts into the packing box 1 from the each second spaced orifice 22 or the each second spaced aperture 33 via the each second spaced hole 12, and the each binding strap 60 extends outwardly from the each first spaced hole 11 through the each first spaced orifice 21 or each first spaced aperture 31 so as to surround 20 the each top casing 50. The each biding strap 60 includes a fastener 61, a male fastening face 621 formed on a top of a distal end thereof, and a female fastening face 622 arranged on a bottom of a distal end thereof, wherein the fastener 61 fastens with the male fastening face 621 and the female fastening face 622 so that the each binding 25 strap 60 fixes the packing box 1. 6 Referring to FIG. 5, the each bottom casing 40 includes plural first extending fringes 41, and each first extending fringe 41 has a first opening 411 defined thereon; the each top casing 50 includes plural second extending fringes 51, and each second 5 extending fringe 51 has a second opening 511 defined thereon; wherein the each bottom casing 40 further includes plural third openings 42, and each third opening 42 corresponds to the first opening 411 of the each first extending fringe 41; the each top casing 50 further includes plural fourth openings 52, and each 10 fourth opening 52 corresponds to the second opening 511 of the each second extending fringe 51. The packing box 1 further comprises plural first fixing elements 70, and each first fixing element 70 is configured to fix the each first extending fringe 41 and has a hollow extension 71 15 which extends through the first opening 411, the second opening 511, the each third opening 42, and the each fourth opening 52. The packing box 1 further comprises plural second fixing elements 80, and each second fixing element 80 inserts through the each third opening 42 and the each fourth opening 52. The each 20 second fixing element 80 includes outer threads 81 formed thereon, and the each first fixing element 70 includes inner threads 72 arranged in the hollow extension 71, such that the outer threads 81 of the each second fixing element 80 screw with the inner threads 72 of the each fixing element 80, thus fixing the each bottom casing 25 40 and the each first extending fringe 41 together or fixing the each 7 top casing 50 and the each second extending fringe 51 together. As shown in FIGS. 1 to 2, as packing a product by using a packing box 1 of a small size, the first side casing 20 and the second side casing 30 are fitted onto the accommodating casing 10, 5 wherein the first side casing 20 and the second side casing 30 are connected together, and a bottom casing 40 of a small size and a top casing 50 of a small size cover the accommodating casing 10, the first side casing 20, and the second side casing 30. Thereafter, the plurality of binding straps 60 surround the packing body 1 and 10 insert through the plurality of first and second spaced holes 11, 12, the plural first spaced orifices 21, the plural first spaced apertures 31, the plural second spaced orifices 22, and the plural second spaced apertures 32, thus connecting the accommodating casing 10, the first side casing 20, the second side casing 30, the bottom casing 15 40, and the top casing 50 together to produce the packing box 1 of the small size. As illustrated in FIG. 3, in a second embodiment, as packing another product by using a packing box 1 of a medium size, the first side casing 20 and the second side casing 30 are fitted onto 20 the accommodating casing 10, wherein between the first side casing 20 and the second side casing 30 is defined a small distance at which at least one first and second spaced holes 11, 12 expose, and a bottom casing 40 of a medium size and a top casing 50 of a medium size cover the accommodating casing 10, the first side 25 casing 20, and the second side casing 30. Thereafter, the plurality of 8 binding straps 60 surround the packing body 1 and insert through the plurality of first and second spaced holes 11, 12, the plural first spaced orifices 21, the plural first spaced apertures 31, the plural second spaced orifices 22, and the plural second spaced apertures 5 32, thus connecting the accommodating casing 10, the first side casing 20, the second side casing 30, the bottom casing 40, and the top casing 50 together to produce the packing box 1 of the medium size. With reference to FIG. 4, in a third embodiment, as 10 packing another product by using a packing box 1 of a large size, the first side casing 20 and the second side casing 30 are fitted onto the accommodating casing 10, wherein between the first side casing 20 and the second side casing 30 is defined a large distance at which at least two first and second spaced holes 11, 12 expose, and 15 a bottom casing 40 of a large size and a top casing 50 of a large size cover the accommodating casing 10, the first side casing 20, and the second side casing 30. Thereafter, the plurality of binding straps 60 surround the packing body 1 and insert through the plurality of first and second spaced holes 11, 12, the plural first spaced orifices 21, 20 the plural first spaced apertures 31, the plural second spaced orifices 22, and the plural second spaced apertures 32, thus connecting the accommodating casing 10, the first side casing 20, the second side casing 30, the bottom casing 40, and the top casing 50 together to produce the packing box 1 of the large size. 25 Accordingly, the packing box of the present intention is 9 assembled in the various sizes based on using requirements. While the preferred embodiments of the invention have been set forth for the purpose of disclosure, modifications of the disclosed embodiments of the invention as well as other 5 embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments which do not depart from the spirit and scope of the invention. 10