To determine the ideal prop for your airplane...

You take the expected cruising speed, multiply by 2.00, divide by the engine rpm at maximum rated power. convert the units to inches. and you now have the desired pitch.

Then there are a series of formulae to find the propeller with that pitch which will use the engine's rated horsepower at the correct RPM.

That will give you the ideal propeller for cruising speed. The engine will be running at appx 75% of rated rpm (trying to pull the model appx 1.5 times cruising speed) and prop slip factor should have you right there. (assuming the engine isn't oversize or undersize...)

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For an average trainer... your expected cruisng speed is 50 to 60 mph. At 11,000 rpm... you want appx a 4 inch to 5 inch pitch to cruise at that speed. (you'll find the math is pretty close, at the 1.5 times speed factor)

To properly load an average .46 ci glow engine using 10% niro fuel... you want a 12X4 or 11X5 or 10X6 prop. Since the xpected cruise speed is under 90 mph... you don't really want the 10X6 (or 10X7 some others will recommend...)

You will find that landings will be easier with the 11X5 than with the 10X6 too. (the 12X4 may give a ground clearance problem.... leading to a lot of damaged props.)