Hi Joe,
With a clark Y flat bottom airfoil and a wing around 24oz. per square foot it should do what you want. Around 80" span and 65" long. This will give around 11 to 1200 square inches area and about seven square feet area. Eleven pounds would give a 24oz. to the square wing loading. The Horizontal stab assembly and elevator need to be about 25% of effective wing area. Ailerons need to be generious, about 2 1/2". Use large wheels (4-5") and a shallow angle of attack on the ground. The engine with a reduction unit would weight as much as a 33cc Homelite chain saw engine and produce much much less power. STOL air craft are usually over powered. The stock Weedy might spin a 18x6 or 20x6 on a reducer at 1.5 to 1. Flaparons are good for this if your radio supports it. This plane would fly ok without the reducer on a grass field. The idea is to keep the wing loading fairly low. Balsa USA makes a very popular WW1 monoplane Eindecker with a 80" wing that has been very sucessful with the 20 to 25cc conversions. Plans are available sepparatly. Good Luck.

Dave