I think it's more important to think about your entire system, and the bio load it will have. The thing that most ppl screaming about tangs don't talk about is the way they eat and poop. Tangs will eat constantly, they process their food very quickly and it is released back into the system very quickly. They are also (not all but enough to make the statement) very prone to Ick. What I would do is design your sytem in this manner
1. Largest tank you - Want to have, afford.
2. What is most important to you Fish or Coral or a mix of both
3. If you want corals, do you want a mixed reef, LPS and Softie, or SPS
4. Using the size of your tank, the corals you want (if you want them) to think about fish (as some great fish are not reef safe)
5. After #4 is considered, fish choices should be based on the following - bio load, swimming room, the place they like to be in the tank (bottom, middle, top)
6. Stocking order (some fish can be very peaceful if added at the right time, however adding them at an incorrect placement in the grand order can result in volience)
I personal agree with the 1st part of Mario's statement in general (70 gal is a starting point) however as a general rule I would say that would be a 1 tang tank. Personaly for tangs I would say min 48" length (they like the swimming room) and as wide and tall as you can go after that.