I use 700 x 28 on both my roadies. The singlespeed only sees pavement use these days, but the brevet bike gets a fair amount of hardpack dirt and crushed/packed gravel trail use. On days when the gravel is feeling loose or the dirt roads are more mud than hardpack, I'd prefer a 32mm or even a 35mm light CX tread; but since most of my riding is on concrete and asphalt, I stick with the 28mm.