For what its worth, I bought a Hippo Tang in early 2002 that was about 1" long and I sold it slightly over a year later when it reached 4" long (That's an increase of 16x in body area in about a year)
I bought another 1" baby Hippo Tang on May 18th, 2003 and I sold it on Jan 7, 2005 (20 months later) when it was again 4" long (16 times bigger).
My tank is 38G. I wouldn't personally keep a fish beyond 4" in a tank that size. Fortunately, my nitrates was close to 0. At one point, I had 2 large fish over 4" (1 Tang, and 1 something else) in there and my nitrates was over 75. After I gave the largest one to EmilyB, I was able to control my nitrates.
Here's a comparison. The large Tang in the first pic started off the same size, as the little Tang in that pic. After I bought the little tang, I immediately sold the large Tang within days. As of today, I have sold the 2nd Tang as well. My tank is now Tangless and looks really bare.
To me, it seems true that healthy Tangs grow very fast and my small tank did not prevent mine from growing fast.
May 19th, 2003 - Large Tang in this pic had grown from 1" to 4" in a year or so. The baby Tang in the pic would eventually grow to 4" as well as seen below.
Here is Tang number 2 (which was the baby Tang from above) 20 months later on Jan 7, 2005
Interestingly, in this time frame, the fish were all healthy and thriving. However, all of the Acros from the first pic are no longer with me (They all died).