Hi!
A 11x5 or a 11x6 (APC )is what you should use, at least if you fly at sea level and going to use it in a high winged trainer like the Kyosho Calmato or a plane like it.
A 11x4 or a 10x6 is not so good for this kind of airplane. but for a 3D type of airplane a 11x4 might be wort thinking of.
Remember this engine is a "puller" ...it doesn't like to rew much so keep the rpm down to between 10000-12500 rpm.
5% nitro is all it needs to be tottaly reliable, 10-15% nitro is owerkill!!

Regards!
Jan K
Sweden