im going to be running a rental shop soon, and i want to print the covers for many dvds provided here,my question is, are the covers provided for download here at a specific size to fit a dvd box when printed or is this something i have to adjust myself?

The printing should not be a problem. So long as it is for legitamate copies and not the unauthorized dupes or bootleg versions. As for the sizes. The actual dimensions print to fit covers for whatever catagory.
So if there is a blu-ray cover it will print that size. The only issue that may come up is with the printer settings. Often a printer will auto set to fit the whole image in the present margins. You may have to adjust this. I know that on my printer at home I had to drop the margins to 0 and fool the printer into believing that it was using 8.5 x 14 paper for a standard size case and the thinpak versions. While the print can fit on standard 8.5 x 11 paper for some reason it only worked if I set it up the above mentioned way. There is also some printing software available in on the site. http://www.dvdcoversfuzion.com/forum/showthread.php?t=254