Haddonis have shorter tentacles (( though once in a while one will have longer tentacles for a day or two for no reason -- at least mine has )), compared to Gigs.
As for sizes, anywhere from about 5" to over 20, though size can be somewhat controlled by feeding less. My 11 year old one has been around 15" for the last 5 years or so.
As for price, would guess anywhere around $50 for a "plain" one to $300-$500+ for a blue and/or red one.
EDIT --- All 5 of mine are in reef tanks, and never had any real issues. I have only lost 2 fish in the 11 years I have been keeping them, and the only corals that I have lost were frags that fell into them.