I have a couple that are very large...no idea how large as the most I've seen out is about 10 - 12", but with more going back into the rocks they hide under.
Like previous posters, I've honestly not seen any problems from them...they seem to keep any decaying matter under control (I lost several fish in one bulk with a power failure, and the bodies didn't stick around long enough to even pull out).
They are creepy to look at, but I'd say they're definitely a valuable part of my CUC....and thus far have not damaged anything alive in the tank.
firefish, ocellaris clown fish
caulastrea, green star polyps, palythoa, zoas