I've used either 25 or 28 mm tires for decades off-road. They are fine until I find myself with miles of either sand or water-saturated clay, then they fail miserably. If someone put down some gravel or crushed limestone, anything works, but not equally comfortably. Now I'm old and no longer get a thrill from fighting the surface, so I roll much fatter tires (up to 54 mm on the tandem). In fact, I rarely go under 42 mm any more. There are some awfully nice supple wide tires out there that, while a bit heavier than the skinny tires you're used to, have lower rolling resistance on almost every surface.