10% of the wing area. You lose some authority at low speeds (if you're a high alpha/3-D flier) by removing them from the slip stream. And if you put all your aileron on at the wingtips you increase the likelyhood of stalling a wing and snap rolling at low speeds, also.

Have you considered flaperons that do both?

I have a Contender that has 12" of apron flap along the wing root - one solid piece below the fuselage and all - on a 60" +/- overall wingspan. Don't know of any formula.