Before I suggested that there be a separate box on a character sheet for a chat avatar, so one could still have a large character portrait, and a smaller picture to use in the chat box, which is easier to make out than a very small version of a large portrait. However, with the new "Default token" feature of character sheets, the easier thing to do would be to just use the "Default token" image, which is usually a smaller image, as the chat icon, which would be easier to make out than a tiny version of the large portrait.