I don't think I have ever seen a size requirement on receipts. Certainly the number of pixels for photos is a general requirement. Many companies do not want the poor resolution of the old camera phones and they don't want the horrendously space hungry photos from a good digital camera. This is not rocket science and I have never had a shop rejected because photos were too large or scans were too large, but then again the reporting software may kick out and not accept a file that is too large.
If you have reasonable photo or scanning software, it generally is not particularly difficult to adjust for clarity, contrast or size. It just takes a little practice. Some of these guys will even accept clear photos of receipts, so no, they are more giving guidelines I believe, than cast in stone requirements.
You seem to be referring, however, to one particular company and if what you are dealing with is the specifications for a pizza shop, those do tend to be pickier than others. Probably even more important on those than the number of pixels is the exact angle from which the photo was taken and exactly what is shown in the photo.