ORIGINAL: jaka
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.
Jaka, you & I really disagree on this one. The torque peak of the 40 LA is @ 12,700 & you need to at least reach that to extract usefull power from that feeble engine. With the power peak @ 13,400 it isn't exactly a rever, but it can't push water downhill with an 11-6, regardless of the airframe it is fitted to.
An 11-4 is a good trainer prop for it, but even with that it can't reach the torque peak on a cold day at sea level.