size is hard to say, its more about age. females are considerably smaller than males and as long as they are pairing off they should breed readily. usually rams you buy at fish stores are just a few months old so they should breed as long as you have the conditions in the tank. other than that if your trying to breed them FOR spawing then wait till about 3 to 4 months.
Cool, I found a good looking pair of females that claimed to be German imports instead of the usual rubbish from Asia. Bellies are very red (hopefully not hormone induced). I introduced them to my big male and they all seem to be getting on swimmingly. Quite a bit smaller at an inch and quarter.