Yep....and that really isn't a stupid question.
Reason: If you want your hermits to grow, you need to provide larger shells for them to grow into. If you don't want them to grow, then don't add any new shells.
Clause: If you don't add any larger shells and you have snails in the tank with larger shells, the hermits may evict the snails by force and move in.
Possible solution: My LFS just gave me some old snail shells they had in a tank, free of charge. Keeps my hermits happy. The funniest thing is watching a little hermit think he can carry around a shell WAY to big for him. Eventually the give up and move back to a more suitable size shell.
..........MCF