I had one in a 125 for 4+ years, but I wouldn't do it any anything less than a 6 foot long tank, in fact in retrospect, it wasn't the best fish for my 125, but he seemed happy enough, he was roughly 7 inches long.
don't do it, I had mine in a 90 and the naso was pacing back and forth constantly. I now have a 240 and the tang is about 12 inches long from mouth to streamers and I don't know how much longer I can keep it in a 6 foot tank.