The flaps that FLYBOY mentioned (90 degree down) work very well for speed control. I would say 50 to 60% of span and 10 to 15% of chord is workable. I have set-up a few gliders this way with mechanical elevator compensation. Building the mechanical mixer is fun and easy. Either mechanical or electronic, you'll need some down elevator mixing to prevent a sharp up pitch.

I have never been a fan of full span flapperons. For gliders, it doesn't provide speed control, deteriorates aileron effectiveness, and makes tip stall more likely. I see guys use full span flapperons at my power model club for no apparent reason. It usually causes a trim change, tip stall, and crash. For a glider, I would stick to the old formula.