How big a prop?
Whatever the flying field and the engine both allow.
Of the three grass fields I fly, my experience with the Raptor was at the closest cut. We went up in diameter 1" and the blades showed green after one flight.
I try to suit the propeller to the engine a bit more rigorously than just "smaller diameter and moderate pitch". I try to find a combination the allows the engine to turn in its best rpm range with a prop that has the most suitable pitch for the airplane to do what's expected and to do it with as much diameter as possible since more fan area beats less every time. That's three different parameters all trading off against each other.