CoosBayLumber,

It is good to know that simplicity is an advantage. It may not be possible to get the kind of performance you are after with a nice simple approach, as has been mentioned. I can't really answer in terms of square inches, as I don't know how much span you have to work with before you run into ailerons. If you make fowler flaps or simple hinged flaps from the root to the ailerons, about 30% of the wing chord, and deploy to 30 or 40 degrees, you will be getting a big chunk of what is possible with a simple implementation. If you are building a new wing, you may be able to give up some aileron span to get more flaps, depending on how the plane flies now. If you don't need to retain scale proportions, you could add some chord toward the root, and have bigger flaps.

Good luck,

banktoturn