The smallest photographs I have seen so far are contacts from 35mm negatives by Kertesz. They are perhaps the pictures which hold each viewer for longest. To see a little picture, you have to bend close to it. When you do that, the rest of the world ( in this case a noisy and busy art fair ) disappears.
Perhaps it can be formulated like this: a picture can either hold the wall, or hold the eye. I know which I prefer .