Read this thread. Also, if you get Donator, your avatar's maximum size will be of 200x250 and your profile picture of 240x240.
You can have a bigger avatar and profile picture by achieving at least 2500 posts, which if you do, it will directly make you a LVL 2 Member and therefore allow you to have an increased avatar and profile picture size. For further information, feel free to check this thread.