Not JUST larger, but OBVIOUSLY larger, to the fish.
My understanding is that all clownfish are born male. If you get a much larger, thus older fish, it will either BE female (if its been around a smaller male) or TURN female (if it is large enough to be obviously the dominant fish). If the size difference is not enough you have a crapshoot on whether they just fight it out to be dominant or pair up. Especially if they are introduced separately.
Changing the arrangement of the tank will cut down on territoriality of the present fish, but what you also need to do is make sure the two are compatible, or can become compatible. Thats the size difference. With your current one being SO small, I would think that just a 1/2 difference in the new one would be enough to make the difference and minimize the risk of the dominance fight.