I've had my desjardini (Red Sea) sailfin for 12-13 years - when I got him he was about 3". He's easily 8"-9" now, but I think the majority of that growth was early on. He was in a 4' 75g for many years, then temporarily (maybe about a year) in a 30" cube, now in a 280g 6' for the last 2 years. I wish I had thought to measure him (or at least do a better job of remembering better his size at the time) at the tank transfers. Intuitively, I feel like he has grown a little in the last 2 years but I have no empirical data to confirm it.
My lavender tang (Acanthurus nigrofuscus) was purchased pretty small. Maybe 3"-3.5" tip to tail. Lived in the same 30" cube for the first year, then into the 280g. He's 7"-8" now, so definitely bigger. But similar to the sailfin with the growth curve, most of the growth was in the beginning and has been more or less the same size the last 2 years now.
My lieutenant tang was a trade-in at J&L. I don't think he's grown much in the time I've had him (2-3 years). Hard to say, he might be a bit bigger since then, but not much. Certainly wider (I feed well too
).
Not a tang, but, interesting to note. In my 280g I have a double-barred rabbitfish (Siganus virgatus). Bought him at a tiny size (1.5"-2"), was the second largest fish in the tank after the sailfin within 2-3 months (8" ish). In my 65g, I have Siganus doliatus (which is very similar to virgatus but more blue stripes). He was 2" at purchase, and in 2 years has grown to maybe 3.5". The species may be different technically but I've seen S. doliatus at public aquariums at 12" long so the size potential is there. So it makes me think that tank size does play a little into the equation.