Are you looking for a certain size because you're worried about the big one beating up the little one while they get used to each other? From what I understand, opposite gender fish usually vary A LOT in size. My lfs has a pair of maroons and the female is at least 3x as big as the male. Some fish can switch gender, too... I'm not sure about maroons doing this, but I wouldn't be suprised at all if they do. I've also heard (and take things people have "heard" with a grain of salt) that once they become female they can't switch back to male again. It would be something to check into first, at least. With my perculas I bought them both together while they were very young, so they bonded and grew up together from the start. Good luck!