Female maroon will get as about as large as your hand within 2 years.
Male maroon will get approx 2-3 inches if the female lets him eat enough.
BTAs normally get 8"-12" before dividing, rinse and repeat growth sequence.
I would not go smaller then a standard 29gal size and you would be much better off with a 30cube+.
fwiw: maroons can be very tough to pair and quite harsh on your decor. Expect the female to move sand, frags, rubble and even some good sized rocks. They can also be very rough on their host anemones, make sure the anemone is 2x-3x times as large as the fish.
fwiw2: when the clowns are mature, you'll be much better off having 2 or 3 BTAs in the tank with them to spread the abuse.
Good luck