I've kept mine in a 500g tank for over six years and, at just about a foot long, he's beginning to look a little cramped. He certainly utilizes every inch of his swimming room and I do believe that had he not 'grown up' (I got him at around 4-5" long) here, he would have destroyed quite a few of his tankmates by now. As it stands, he's active but mellow...gets a bit fiesty at feeding time though and will run off the other tangs until he gets his fill at the nori stick.
But to answer the question, I wouldn't put a Sohal in anything less than an 8 foot tank unless you weren't planning to keep it until maturity. I would be careful with my aquascaping, leaving lots of swimming room and giving it several good places to dash when startled. These places are also good for the other tank inhabitants to flee should the Sohal get PO'ed for whatever reason. They are amazing quick and when angry will dart back and forth through the length of the tank a few times, scattering everthing in its path. It would be nice to give these other fish someplace to hide, especially the more skitish ones. Also,make sure your skimming or filtration is up to snuff because Sohals can eat and poop in epic quantities. And finally, if at all possible, I would also add the Sohal last. Waaay last. Hope this helps a little.