AlexF was thinking of the Nexstar airbrakes but I don't recommend them. I would stay with a 10x6 or 11x4 for good performance and landing speeds. That 8 pitched prop is all wrong IMHO for a trainer and it is going to be pulling too hard at idle and harder to slow down for landing. You could get a longer nose leg if needed like the ones made by Fults, good and strong and take to rough fields well. Variety of lengths available. Then rebend the main gear slightly to match the new height for proper stance.