I would say 5 wpg would be enough. As said, xenia is a light hungry coral. A kinda fun, or kinda annoying thing about them (depending on how you look at it) is that they will migrate up to the top of the tank. Given enough time, it will move up as high as it can. Time and time again, I've put xenia somewhere in the tank, and it almost immediately starts moving up the rocks. Xenia can't actually move like an anenome, but the way that it grows and spreads results in it moving up the rocks. My opinion is you may as well just put it where you want it on the top of the tank.