Well, your question’s in inch so I’ll tell the answer in approximate inch. Usually there are 1200 x 1800 pixels in such a photo at 300 ppi. The film’s has about 24mm x 36 mm to place a picture. In the US 4×6 is the most popular photo size because as I said before, 2:3 ratio format is perfect. In Japan, for example, the common size is 3.25 x 4.5 or 3.5 x 5. All these print formats came from 70-80’s with the invention of Kodak film.