Question: Is it OK to change to smaller or larger diameter wheels? Any tips on this?

Answer: Depending on the model, the stock wheel diameter has been chosen to work well on asphalt, packed sand/dirt /&or close cropped grass. Yes it is OK to change the diameter of the wheels provided that you maintain the same angle of incidence when sitting on the ground... i.e. if you change the diameter of the mains you must change the diameter of the nose or tail wheel by the same amount and vice versa.

Here are some tips:

1. The angle of incidence on the ground should be the same as that provided by the stock wheels to ensure that the model has the correct angle of attack when making its take off run and when landing. i.e. if you change the mains by X inches in diameter, make sure you change the diameter of the nose or tail wheel by the same amount and vice versa.
2. If you are always flying from smooth asphalt, concrete, clay or other very hard surface, you can reduce the wheel size slightly provided that you retain sufficient prop tip clearance from the ground. You may have to opt for a smaller diameter prop with a larger pitch or more blades to sustain sufficient ground clearance for the prop.
3. If you are flying from packed dirt, fine gravel, sand, sandy soil or closely cropped grass, the stock wheel size is probably best unless you have some particular need to change the wheels.
4. If you are flying from a grass field where the grass is quite long and/or the turf is soft or spongy or variable in "hardness" you may need to go to larger diameter wheels to get consistent ground handling and stability. If your model appears to be bogging down on take off, failing to run out smoothly upon landing or is hard to taxi in a straight line try increasing the diameter of the wheels in .25" increments. Use lightweight wheels and test as you increase the diameter in .25" steps. Generally trike gear will require larger diameter wheels in these conditions than tail draggers due to drag on the nose gear wire/axle.