Ausitn,
Re-read Dukes first paragraph. That's a good rule of thumb, (and Larry) it will help explain why an 11 X 7 is being considered too much prop for your engine, but why an 11 X 5 isn't.

Remember, the Diameter of the prop determines how much thrust you're going to generate (as in vertical pull and the ability to take off more quickly), while the Pitch determines your overall speed.

ie. A 10 X 7 on a 40 or 46 engine will produce more speed than an 11 X 5, but take off runs will be longer and the plane will not slow down as well for landings.

Selecting props IS a bit of a black art, but the best way to do it is to look at what the engine manufacturer recommends, then selecting for either speed or thrust.

One other point of interest. The pitch of the prop is theoretically the distance in inches that the prop will move forward with each revolution. For example, a prop with a 7 inch pitch will move forward 7 inches with each revolution (or 5,833 feet per minute), while a 5 inch pitch will move 5 inches (4,167 feet per minute). Of course that doesn't take slippage, drag, and friction into account, but it does giv a comparison.

Good luck, have fun, and welcome to RCU!
Dennis-