If you want to scale a plane up or down and have it fly similar to the original design, make sure that the wing loading and power loading match the original design. The wing loading is the total model weight(tank empty) in ounces, divided by the total wing area in square feet. To convert square inches into square feet, divide the wing area in square inches, by 144 sq. in. per sq. ft. So, for a plane with 600 sq. in(4.167 sq. ft.) wing and a weight of 5 pounds(80 ounces), the wing loading would be WL = 80 / (4.167) = 19.19 oz/sq ft.

The other scaling factor, power loading, is the total plane weight(tank empty) in ounces, divided by the cubic inch displacement of the engine.
So, for a 40 size engine(0.40 cu. in.) (CID)and the 80 ounce plane, the power loading would be PL = 80 / 0.40 = 200.

Now, for the new design, just work the above equations backwards to find the wing area and CID of the plane, starting with a wing loading of 19.19 oz/sq ft and power loading of 200.

If you have an electric, you could redefine the power loading factor as wing loading divided by power in watts, that is, voltage times average currentfor example. I'm not into electrics so you should check the electrics documentation to see if they use another definition. You may also find other helpful information at www.rcaeronauts.com.
Best of luck.<br type="_moz" />