The coldest I have actually ridden in was a -4F/-20C air temp, with windchill that made it a -22F/-30C. For that you simply can't wear cleated biking shoes. It is far easier to find leather winter boots with insulation in wider widths than to find wide women's biking shoes anyway. My normal shoe size is a 61/2 W, that day I wore 71/2WW boots with a pair of thin wool liners and a heavier pair of wool socks. After 40 minutes of riding, my toes were starting to get cold, but not the tingling, burning cold. Mini toe clips make this a fairly efficient setup. Good waterproofing helps keep your feet dry from snow. By the way, the same day, wool chopper mitts with leather overmitt kept my fingers equally happy.