Gravatar automatically resizes images anyway, so the original size makes no difference. Just make sure that you select something which will still be clearly visible when it's made small. There's no point is choosing an image which has too much detail. For example, this would suit your username rather well but people wouldn't easily be able to see who it was in a very small version:
http://tinyurl.com/plcuuc7
(Sorry. I just couldn't resist that!)
;-)