I just cut off the inside of each elevator half until I got the max throw out of the rudder that my servos and pushrods would allow. I got a lot! It will Knife Edge...

The counterbalance on top of the rudder makes a much bigger difference, compared to making it longer. I made the rudder 2 inches longer on one of mine and it would not KE. More throw, a counterbalance, and a rearward CG makes a big difference. What it really needs is a taller fuselage!