The reef locster is currently 3/4 in long. I've read that they get about 14 inches. But I know someone that has had one for 3 years and it's only about 4 inches in length. That brain didn't make it. It RTNed had to pull it out before it polluted the water further.
I had one of those about 12 years ago in a 55 gallon, he got way out of hand at night. Mine grew quite fast, from 2 inches to about 5 in 6 months. wait till he molts, you will think he died!!