What do you think the optimum tank size is for a collection of saltwater fish? I am thinking about getting a larger one, but I don't want to overwhelm the fish.

You may be happy to know that the larger the aquarium, the easier it is to maintain. Smaller saltwater aquariums are harder to keep b/c changes are felt more by the fish and they are harder to keep clean. Having a large tank won't overwhelm fish-just think how large the tank they came from was!