i've seen one in my lfs that was 9 to 10" , it was easily the biggest one i've ever seen. i've never had much luck getting them to grow, they seem very slow growing to me but it seems like other people are heving better luck at growing them.
Mine didn't grow at all in the like four months I had it. It was small like 2". My friend though has his right for about 6 months now maybe and they have grown like 2-3".
My male golden saum is almost 2 yrs old and just north of 6inches, my female (same age) is 5 inches. Largest I've seen (LFS) 9 inches and quite tall (6-7inches).
Mine spawned at 4 inches and 3 inches (male/female).