Achilles, there is no automatic resize option on ForumUp forums (or on phpBB as far as I know).
From experience with YaBB I can tell you that it is actually not such a good idea, as it resizes and distorts all Avatars that is not of the exact same size, and/or the exact same height to width ratio, as the setting.
This means that people end up having to crop instead of resize, and for a more obscure reason, and it often becomes impossible to use small-ish animated avatars, as the resize "magnifies" them to blurry pixels.
Scoobs is correct about the URL. Thanks Scoobs.
FoxLeonard