I cannot seem to change the text saying the maximum image length is 150 pixels but here's an announcement to let you know that the maximum length is in fact 200 Pixels by 200 Pixels.

Only the forum won't allow you to pass 150 Pixels in length if you're uploading from your Computer so to get a bigger avatar you'll need to use an Image Hosting website such as Photobucket.

If you're having problems or would rather someone else do it, just post here or PM me.