Everyone is going to criticize the following comment, but I will go ahead and state it:
Avatar sizes should be allowed up to the width of the column. The height, however, should be limited. Why? A wide avatar limited by the column's width will never disrupt vB
layout while a tall avatar could cause loss of space. I think maximizing the size of avatars allows users much more freedom when putting up pictures. Their avatars can be more descriptive and won't make other users have to squint at it to read what is inside. Unfortunately, this forum will never operate under such conditions, so I hope that the 100x100 does get implemented. And, if you couldn't tell by my stance, I am fine with Thelis' avatar so long the height does not cause an absurd amount of wasted space between replies and so long the width does not create uneven "user info columns".
edit: Moments after posting this comment a realized that in order for a descriptive image of higher dimensions, users would need an increased in memory size allowed. I don't know for sure, but if your pictures are heavier it would seem to me that the pages would load up slower for those without the privilege of high speed internet. With this in mind, I void my earlier argument of self-expression.