As a rule, Wieland says, players' mouth guards should fit tightly to their upper teeth. They should be able to talk without the guard falling out.
If the guard is too loose, it can become dislodged during action and cause the players to choke, or come out of the player's mouth, leaving them unprotected.
All parents should check to see that mouth guards fit properly.
If you need some guidance on that process, please visit this website recommended by Joe:
http://www.wikihow.com/Adjust-a-Football-Mouthpiece
