How can I make it so that if a player has one of those small avatars like the one in the image, it will force it to look regular in the game? Like it would have the pancake as the head but then the rest of the body would be normal blocky?
You should be able to do this through the avatar settings. I don’t know exactly how to fix this but I know there’s many resources on the devforum already though.
You can go to your game settings, then navigate to avatar settings. There you should find an option to make your game r6, this will barely change anything except that you should make animations on an r6 rig (i think), but it will remove these custom characters.
Hope this helps 
There’s not really any good ways to detect these avatars without creating an enormous list of bundles and checking each limb to see if its in a disallowed bundle.
Avatar settings do not help. Use R6.