A sort-of related idea is: How small can an image of a human face be and still be recognizable? I remember X-face images from mail headers years ago -- these were just 48 x 48 x 1 pixel images, i.e., just 288 bytes (yes, bytes, not KB or MB). I marveled that the faces were indeed recognizable. I wish I could find an example of an X-face of a famous person (or even an unknown person) to give as an example here, but this seems to be one of those things that are ungoogleable.