Just to make it perfectly clear...
If you upload an avatar which takes up half the screen, is offensive or of an explicit nature you will be suspended for a week - no questions.
We are quite forgiving on the size of an avater at the moment and we don't want to start making it a set size so please read the above and read it again!
If you are uncertain about the size which are allowed, then look at someone elses' and make it smaller than that!