We currently have a 4x2x2 and have one very fiesty yellow belly regal tang in there. She came with the tank, and is around 12cm from head to tail. Aware that we need to rehouse soon (getting a 6ft tank soon) but she seems pretty happy for now tbh.
The tang debate seems to be hotter on some forums more than others, a few I looked at before joining here made me a bit wary of asking anything regarding my tang...with the overzealous tang police on patrol you'd get shot down pretty quick! Of course, not saying that some of the points on space aren't valid, they are and it's true tangs need space to swim, but there are sensible options available to keep tang and tang-keeper happy :)