What size propeller for a 36 V DC motor?

Does the forward push on an ordinary electric motor cause abnormal
wear on the bearings? Or maybe that is countered/supported by the
magnetic part of the motor?

Very good point. The bearings do take all of the load, called thrust, and it
depends on the type of bearing whether it can stand the end load. Most likely
that motor would not do well. It would be best to use a geared adapter to turn
the prop, and that will remove the thrust load from the motor. Check model
supply places for a suitable gearbox.

I seriously doubt that you are going to get more than 8 pounds of thrust out of
that motor, even with the ideal gearbox and prop. Probably not going to do well
to move a person on skates.
--
Jim in NC