the best way to get an avatar that's just the right size is to make it yourself, or have someone do it for you --
this can be done a couple ways --- if you have a graphics program (don't know the titles of any, b/c i don't have one) - you can make them -- or if you have microsoft photo editor - it will do it too (guess this is a graphics program) and i think 'paint' on typical microsoft based compies will do it too --
what you do is find you're piccie -- (and this is in MS Photo Editor) -- open it in the program - 'crop' it as necessary - cut pieces off to include the part you want as your avatar -- then 'shrink' it --
you kinda just have to play around w/ it until you get one that is both 50x50 AND the right file size -- b/c they can only be so large or it would take up too much space ---
if you can't figure out how to make one, or just don't wanna - post a note in the chit chat thread and someone will be happy to oblige you ---- there are tons o' ppl over there who love to do stuff like that -----
hope this helps!!