IMO a minimum of 29/30 gallon tank is required. How big they get depends on water, space, lighting... My first one started out softball size, but as the colony grew they tended to get smaller. Depends on the variety you get.
Well, then I might have to re-think an anenome then. I have been planning on getting a 29, but I don't want there to be a chance that something does not have enough room.
Thanks for the reply.