NWT Size 6 Thermoball Mule Slippers. Stay cozy and comfortable in these stylish slipper mules from The North Face. The gray heather color and mule style make them a perfect addition to any casual outfit. With a US shoe size of 6, they are the perfect fit for a variety of women's shoe sizes. The ThermoBall Traction V model provides warmth and comfort, perfect for lounging around the house.