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 .
Francis Hodgson 
Francis,
I believe a picture (if good enough) can ‘hold the wall’ and ‘hold the eye.’ The size of a photograph is less important than the strength of its content.