I have a black brittle star about that size, maybe a little bigger. He was in my tank for a little over a year. When I got him he was about 6" across. When I moved about a month ago I finally got to see him all stretched out, and I was shocked to see how big he's gotten. He hates light so he never fully comes out of the rockwork, just sticks an arm out here and there for food.
Hey Kelly, there's actually a difference between what you're seeing in the picture and a serpent star IMO. Serpent stars are smoother and have protrusions on the sides of their arms, or none at all. They tend to be more predatory, and will go after fish and other inhabitants. The starfish above is a brittle star, which is not nearly as aggressive, and will not reach out and grab a fish. They are similar in body structure and habits, but not in aggression as far as I'm concerned. Not a scientific opinion, just what I've picked up through the years.
I have what must be a pretty big one living in my yellow scroll coral (must've hitchhiked when I bought it). Kinda funny, whenever I put food in the tank you see these five arms come waving out the crevices! I thought maybe he was trapped in there by the coral growing, but just the other night I saw he moved to an adjacent rock. He must go for nightly strolls, because in the day he's always in the coral.