When I bred my fader FH pair, the male was around 10" and the female was around 5"/6"...... I hear that's a good size to make them pair up as females tend to be very aggressive during spawning and a relative size difference helps the male to cope up. There are exceptions of course! 
My current pair is: male = 9.5"; female = 6.5"
My current pair is: male = 9.5"; female = 6.5"