I have read that yellow tangs get to be 8 to 10 inches. But I have never seen one larger than 5. Could you post pics of larger ones that you have and there size.
As with most saltwater fish,,,It is not the actual size of the fish that limits the tank size.
But it's teerritorial,and swimming needs.
Tangs require larger tanks because they tend to claim LARGE territories,and need alot of room for swimming.