LT40 plans call for a 10" prop on a 40 motor. I believe it is recommended to be a 10x6 on the plans, but I don't have them in front of me.  The short nose wheel on the Kadet will give you clearance issues in grass with anything bigger than a 10.  I have an 11 on mine, but I turned it into a tail dragger with 4" wheels, so grass clearance is a non issue, but when I laid the motor and prop on the plans, there was not enough clearance for grass T&L.