SLIGHTLY smaller is ok. Larger is even ok if you run through tall or thick grass on your runway. The plane should sit level or SLIGHTLY nose down. NEVER nose up. If it sits nose up, it will tend to take off too quickly (stall, snap, crunch) and be hard to stick on the runway when landing. If it sits too much nose down, it will take a lot of elevator to horse it off. When it does come up, it will be quick and in a steep climb.

Dr.1