I would assume it doesn't matter and they go by volume in a way. If the water is 6" deep but has a vast surface area, they might not grow all that big based on how far up and down they go is what I would think.
Regardless, they need a new home and you're going to need a proper aquarium. There's no getting around that. And do it soon, they've probably been in the bowl for too long and are stunted.