I'm a big believer in BIG wheels.
3" or more if you can for grass.
I will run the biggest tire I can. Helps with take off and landing and helps me with orientation in the sky.
I have these 4" tall (but narrow) wheels I picked up somewhere that I like and I also have some 3" foam wheels that I also like other than the grease and grime won't ever come completely off.
My 2 trike planes have something like 2" wheels on them that I see a HUGE difference in grass handling. (I'm switching these to bigger wheels this summer)