Back in 2002 or 2003 I bought the smallest one I had ever seen from Old Town Aquarium. It was about the size of a nickel. For several years it ate everything except pellets. Then after about 4 years it started eating pellets too. Grew quickly to about an inch and a half, then slowed down. Now, 12 or 14 years later it is around 2.5". I have never seen a "big" one.
It doesn't like new additions that are about the same size as it. But after a few days it leaves them alone. It has never been around corals in my care, just live rock.