Mr. Big is shrinking — and putting on muscle. Chris Noth (pictured), Carrie Bradshaw’s on-and-off lover in “Sex and the City,” is at Canyon Ranch in Lennox, Mass., exercising and eating healthy. “I just finished 10 miles of cross-country skiing in 10 degrees below zero. You’ve got to love the cold,” Noth told us yesterday. When the countdown ends tomorrow night at midnight, Noth said, instead of celebrating, “I’ll probably be in bed exhausted.”