speed 125 km/h

Your Tractor Aircraft is flying at max speed of 125 km/h.

3/ Choose here the diameter of your propeller =>

max diameter : 160 cm	max diameter : 165 cm	max diameter : 170 cm
max diameter : 175 cm	max diameter : 180 cm	max diameter : 185 cm
max diameter : 190 cm	max diameter : 195 cm	max diameter : 200 cm
max diameter : 205 cm

The diameter of a propeller depends on the ground clearance, i.e. the distance between the ground and the tip of a vertically oriented blade.
The ground clearance of ultralight propellers is usually bigger than 20 cm.