It did kind of get missed didn't it....... Sorry.

For a good development model look to sailplanes for guidance. Most of the current sailplanes use anywhere from 17% to 22% of the wing chord for width. So for your friend's 6 inch chord this would be between 1 to 1 3/8 inch flap chord. And the span should be as long as the ailerons will permit. Not sure how big your ailerons are for span so the final sq in's will depend on how big the flaps are using the percentages above.

For max effect it can also be benificial to droop the ailerons as well. But to help prevent tip stalls and adverse yaw effects keep the aileron droop to much less angle than the flaps. I had a flaperon glider and the coupling got a bit messy when the flap deflection went past about 20 degrees. More than that and the model didn't want to do anything aileron related properly. So if you try the airleron droop only let them come down a max of 10 or 12 degrees when the flaps are down to 45 or so. If the flaps go further then leave the ailerons at the 12 degree mark. You'll probably need a computer radio for that. In so small a model it's not really practical to carry all the servos to make this work.

Flap deflections are much more easy. Takeoffs will probably work best with 20 to 30 degrees as a good compromise between lift and drag. Landings can use a full 90 degrees for very high drag and high lift.

This is a pretty small model you're dealing with. It's still a great experiment but don't be surprised if it isn't all you hope for. A 6 inch chord wing running at the speeds you're hoping for is running at a very low Reynold's number. This means the old scale factor may just bite you and not let the flaps perform as well as they would with a larger model.

Good luck and I hope this helps out.