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Are you going to discard the DVD boxes? Why not just cut the labels to size? You'll lose some of the imagery, but, you can choose exactly how to trim the excess.
If you have access to a photo copier, it could be a solution to shrink the DVD label to a specific size as part of copy process.
Do you have a scanner and printer? One option would be to scan the DVD labels and print them to a desired size. You could also take a it USB drive with scanned DVD labels to FedEx Kinkos and have them print them to a desired size.
A more costly option would be to take a stack of DVD labels to FedEx Kinkos and have them do all the work.