http://reefcentral.com/forums/showthread.php?t=1946079
The recommended tank size by the experts on this site is 75. Note that's a minimum, not a size for a happy tang. If you haven't kept tangs before, I wouldn't try it in a tank less than the minimum. Not only will the fish not be happy, but you'll also be annoyed by all the pacing and aggression. Speaking from personal experience here.
Something else to keep in mind is whether or not you've quarantined all the other fish in there for Ich and other diseases?