RE: WHAT SIZE FOLDING PROP FOR E-FLITE 400 INRUNNER
The shaft size is 2mm. The motor is not reccomended and was never designed to be used for direct drive with the shaft that small. You need to get together with someone with experience in electric planes to help you with motor/mount, gearbox-prop setup etc. as these issues are detailed and tailored to the plane. The local hobby shop may be able to help you. All parts of the power system battery, motor, gearbox, prop, speed controller must be matched to work together with the plane or you will have major problems.
Edit-This post was not in reply to Dr. Kiwi (I would be speaking to choir). I clicked on the wrong reply by mistake, the reply was meant for the thread starter.