If its a Royal Blue Pygmy Angel, then something 40-55 would be good. I've never actually seen one of these. Cherub and Flamebacks, but never a Royal Blue. Some places might quote smaller, but when fully grown, it will appreciate having a little more room. Bigger will help keep things stable for a reef tank. Just remember that angels are known to nip at corals...not guaranteed, you never know until you get yours and see.