Just like all sea stars, they are sensitive and need a stable environment. When they're not happy their arms will curl up.
All they need is some live rock in an established/stable tank. As far as their size is concerned, I'd say 5-6" span is the norm in aquariums. Unfortunately, many sea stars lead short lives in captivity. I've had mine for about a year and a half now. It's around 3.5-4".