Usually, the specifications of photos are expressed in inches, which is different from the diagonal length of products such as monitors. "A few inches" in the photo refers to the length of inches on the long side of the photo.
For example, a 6-inch photo is a 6×4-inch photo. There is also a commonly used method to indicate the size of a photo in the world, which is to take the integer value of the short side of the photo and add the letter R to indicate it. For example, a 6-inch photo, the specification is 6×4 inches, which is 4R.
ID card, physical examination form, etc. Most of them are 22mm×32mm small, the photo size of the second-generation ID card is 26mm×32mm, the ordinary one-inch photo is 25 mm× 35 mm, and the photo size of China passport/travel permit is 48mm×33mm, in which the head width is 15mm ~ 22mm and the head length is 28 mm ~ 33 mm.
Baidu encyclopedia-photo size