Actually, you are asking a funny kind of question that is being answered in two different directions. I am assuming you are asking how big of a tank for the fish to be happy... 55 gallon. I am going by pg 400 Marine fishes, I assume they are not very active swimmers however they DO get to 8 or 9 inches...
Considering all of your other fish play a role as far as bioload. But you can rest assured that if you get a humuhumu, you will not have other fish for very long they get increasingly aggressive as they get older and they get down right nasty as adults.