not much bigger than the size its at really...(tey usually die or get eaten before they can grow)different kinds can get huge though. mine has about 12 inch long armshow big can they get as my wife is all excited she has a little starfish as I told her she was unable to have a proper one
They are great! They can get picked on, but if they survive they are great clean up crew, you will generally see their legs emerge during feeding in an attempt to nab something with their little hairy legs.