A standard wallet-sized photo is 2.5 inches by 3.5 inches. This photo size comes from the fact that the photos are designed to fit in one's wallet, and they are similar to the size of a credit card.
Some wallet-sized photos are die cut so the corners are rounded, and the dimensions are reduced by 0.12 inches on each side. Wallet photos are typically sold in sets of eight duplicates, and can be offered in a variety of finishes, such as matte and semigloss, much like any other size of photo. Some developers also sell wallet photos in sets of two.