The one aspect about stocking marine aquariums that I have found the most conflicting information on is what is a proper stocking plan when keeping tangs. A part of the problem is the amount of variation between maximum sizes, growth rates, and the temperament / personality of the fish. I have personally seen yellow tangs reaching a maximum size 10 inches and other reaching a maximum size of about 14 inches, both being kept in set-up that would allow for more growth.
So what is the right answer ?
Do your homework, get as much first hand information from trustworthy people as possible.
If you are new to marine aquariums, error on the side of caution and get a aquarium that is a little larger than what you could stock your tang in.
Use extreme caution when keeping multiple tangs in one aquarium in numbers less than 5.
Always watch carefully for signs that your current stocking is not working out in the best interests of the fish.
The below links will provide you with some further information to help you understand this better