I've got two of these in my tank. Originally one, but I'm guessing a split occured. The base of the big one is about the size of a half dollar, and when the tentacles are out, about the size of a silver dollar or so. The interesting thing I've found with these is 1.) I have never fed them, they were hitch hikers. 2.) They only come out when the lights are off. I never see them during any daytime hours. They fully extend at night and I can see them when I put the flashlight on them. Very neat, but strange indeed.