What size motor you use depends on what level of performance you want.
Start with the all up weight.
50 watts/pound = It will fly - good for a back-up motor for a slope glider to save it when the lift disappears or for casual flying when the wind changes directions.
75 watt/pound = Fun sport flying and aerobatics
100+ watts/pound = strong climb and may go straight up.
150 watts/pound = unlimited straight up climb.
All depends on what kind of performance you want.