Mr. Dennis,
I can't recommend any wheel size for 1/2A on grass. I can recommend getting door mats, rubber mats, cardboard, or corrugated sign plastic for a runway. At least the take-off goes well enough. However, the landing always looks like a gymnast performing floor-tumbling at the Olympics.
I keep 6 pieces of rubber diamond plate about 1/8" thick by 2 feet wide and 3.5 feet long for my "on-grass" runway stored in the trunk of the car.
I know...not what you were looking for. But it is an option.