The site will automatically resize the avatar, so dimensions aren't all that important. What is important is to save the picture you want to use as a Jpeg. Other formats don't work.
But what I'm looking for is what I'm looking for is what size the site resizes it too so I can just make the avatar that size in the first place.
You can do those sorts of avatars at Fukufics.com. Several members have shifting avatars. Including Rei who popped up here for a time. I've been trying to get other people there to show up here, cause here's more active.
I forgot he used a familiar here named Ebony. Didn't stick around long, but Rei is always disappearing even at fukufics.com. I had to check my PMs to see what name Rei used here.