If you go to the option of updating your avatar, the specifications can be found there - the maximum size of your custom image is 80 by 80 pixels or 19.5 KB (whichever is smaller).
If you would like to upload an avatar, you can do it by clicking on User CP then selecting Edit Avatar from the list on the left-hand side of the screen.
I hope this helps, but let me know if you have any difficulties
