I like 100x100 avatars. Standard icon size, and nice and square. Typically they're of some anime or video game character, depends on my mood and what I'm really liking at the time. The past few times I've made icons, I typically get a bigger image and crop the 100x100 image out of that. I play with the colors a bit, and then, volia! Simple, lazy, and not very impressive avatar. :)
In the past, the avatar's almost always been linked to the banner in my signature, but I haven't used a banner in months now. Honestly, I like my signature small. Although it doesn't matter because now all signatures are turned off, so I can't see it.