100 x 100 wouldn't do that much harm, Wayne. Surely 10 pixels by 10 pixels more to the avatar space isn't asking too much???
Hm.. Maybe, but I'm sure somebody can make a nice calculation about how much extra bandwith thread is going to generate if an avatar is 100 pixels bigger in size not considering the colordepth.
Anyway, if it helps keeping this site online, drop the avatars completly. That should help bringing the cost down :-)