I thought about just using a css box-shadow on whatever avatar they have, but the browser support isn't nice enough yet. It's actually easier to just not show the container if they don't have an avatar, but I prefer to leave it there anyway.
I've been doing something similar on another site of mine for years and I've found that it both entices users to set an avatar, and also to use the correct size.
I get to cheat and go over the limits because I'm special like that.