I'm all in favour of stopping big avatars, but mine has to be hosted externally, for example, because of the file size, rather than the dimensions. If you remove the option for external ones, can you increase the file size limit while keeping the dimensions constrained?
As I plan to unleash AVATARMAGEDDON soon, 20k would be nice. I think it is 10 at the moment. These things get cached, so the extra file size doesn't really strain the web server much at all.