I had a friend rig a bike for riding on beach sand in the Los Angeles area. He built a frame with an extra wide front fork and rear. He then laced two MTB rims together on long front and rear axles and installed two fat tires on each wheel. The extra width gave stability in loose sand. He also rigged a jack shaft or 5 in 1 reduction gear in the drive train for a low that went down to about 3 gear inches. I saw him pull his infant son up a 35% grade in LA with that set up. For sand yes fatter the better. If there are no rocks reduce the pressure too.