It doesn't actually show the size limit on the avatar upload because it can be misleading. The forum has a limit, but, that's checked only after the uploaded image is processed and resized. I've seen fairly large files (on my PC) get accepted as avatar images because the resize does a lot of compression on the forum end, and the resulting size is much smaller. It sometimes work opposite though. An image that doesn't compress well may get rejected even though it seems small at upload time. Depending upon level of member account, (level being based upon time as member and post count), the "final" (compressed) avatar size limit is between 25KB to 45KB.
