I think it should all be automated on a gradual scale instead of being all-or-nothing. Your avatar size can be automatically maintained proportional to your post count, with a rate of 10 pixels per post. After your first post you can have a 3x3 pixel avatar. After 10 posts you have 100 pixels to work with, so that's 10x10. After 250 posts you have 2500 pixels, or 50x50.
You could set your avatar to any image within the size you have currently earned, and set it to stay as is or scale proportionally larger as you post.
Mr. Anderson's avatar would be 452x452 pixels! Hmmm, maybe that rate is a bit high.