If you put avatar on third party server and use link thare is not such limitation. But you must still to make in not to wide to not break forum tables. I noticed that Solfius avatar is bit to wide, but I will not complain him about it so much... Maybe others will do
EDIT: I think that avatar on 3rd party server should be keep 140 pixels wide max, to not break forum tables. And if animated then should be optimized to lower number of colours. Mine is optimized by my friend to about of half of orginal size, so it will not eat transfer of those who have low speed connection so much.