Just follow the guidelines on the My Profile page (quoted below):
Avatar images will be resized to 48x48 pixels. The maximum file size for avatar images is 10kb.
USAGE POLICY: We expect members who upload avatars to adhere to the community standards for this message board. Anyone who uses this feature as a means to disrupt the message board may risk losing their avatar upload privileges. If your avatar is judged to be disruptive, it will be deleted. The administrators have final authority over what is considered disruptive.
*end of quote*
The My Profile link is at the very top of your DU page.