I REALLY want to change the size of the avatars and I have no idea how. I would also like to disable the ones that the forum provides so that users are forced to upload one using a link or from their computer.
If you want to disable avatars you could just delete the ones you don't want and they won't show. To change avatar settings, go to Admin Panel > Attachments & Avatars and edit the size there.
Easiest way to prevent users from choosing avatars that come with the forum is to just not give them the "Can select a server-stored avatar" permission. Deleting them also works, but there's really no point in giving users permission to choose a server-stored avatar if there aren't any to begin with.
As far as avatar settings go, your best option is to choose to resize oversized avatars (if you allow avatars from a remote site, make sure you choose "download and resize it" - the "Let the HTML resize it" option never has worked properly and will cause all avatars to be resized to the max size even if they're smaller than your maximum size).