For prints, I've found the most common frame sizes are 8x12 in Europe (the standard digital camera output proportions) and 8x10 in the US. I offer both. My photo lab actually has both options - 20 x 30 cm and 20 x 25 cm - so I crop in Photoshop and they print it to the customer's choice of size.
For other types of prints - resumes, catalogs, etc. - A4 is the international standard, but USA has "Letter size" (and Legal, and something else that's even longer). Again, I offer both - let the customer pick the size they'll be printing on.