Calculating thrust and required propeller size for a given engine power
This that follows isn't an accurate calculation, but may be useful as a starting point: let's say the mass of your plane is 23kg. That's a weight of 225 newton. You have to add 830 N for the pilot, so the total weight is 1055 N. Let's assume, also, that the best L/D of your airplane is 9 at 36 km/h = 10 m/s. In a glide, that would mean a sink speed of 10/9 = 1,11 m/s. The implied 'gravitational power' would be 1055 x 1,11 = 1171 watt. That would be the minimum power required for s/l flight at best L/D and for a 100% prop efficiency. In the real world you might need at least three times that power, around 3500 watt.

Now, concerning the relationship between prop size, power, and thrust, at 100% prop efficiency, you may use this formula for the stationary case (airspeed = 0):



(From Wikipedia entry https://en.wikipedia.org/wiki/Disk_loading). Where P is the power in watts, T the thrust in newton, A the disk area of the propeller in sq. meters, and rho the air density in Kg/m3, around 1,22...