A very general rule is that the each flap would be 1/3 the length of it's wing half and the rest aileron.
You would have to increase the throw of the aileron to get the the same performance as before.

On that non-scale aircraft, I would personally suggest not using flaps, but installing a separate servo on each existing aileron and plugging them into different channels. Then, use your computer radio to set up flaperons. A whole lot easier and works just as well.

Highflight