Strip ailerons are typically 7 to 12% chord for trainer to sport models. Obviously the new 3D movement has taken this to extremes but that's a whole other deal.

"Barn door" or conventional outboard ailerons are from 15 to 22'ish% of the chord and from 1/3 to 1/2 span.

If you're looking at adding working flaps then going for the 15 to 20% option and just make the inboard area the flaps is a good way to go.