Look Out!!!! Duck!! INCOMING!!!!!!
Sorry, just anticipating the replies this thread may generate. In general, long term, a 50 gallon tank will be too small for any tang. That being said, you can certainly keep a 2-3" tang in a 50 gallon tank happily for some time. Just recognize, you will probably eventually need to find it a larger home.
Different tangs grow at different rates, and as much as some may contest this, tank size (IME) can inhibit growth. I've had a 3" purple tang in a 65 gallon tank for 3+ years, and it has grown very little (and yes, it's well fed). I have no doubt that if I moved this tang to a 100+ gallon tank, that I'd have 6" purple tang within a year.
Keeping the above in mind, torpedo-shaped fish (such as the regal) seem to appreciate more open swimming room than the sailfin variety (such as the yellow or the sailfin).
<stepping into my flame retardant suit>
Kevin