Small: 3.5 X 4.5 inches. Fits up to women's shoe size 8, men's 6.5
Large: 4 X 5 inches. Fits up to women's shoe size 14.5, men's 13

Thickness refers to the denseness of fur and heaviness of leather. All of our foot warmers are very effective. We offer the following choices based on the amount of room in your footwear.
Regular Foot Warmers are the balance between Slim and Thick and generally fit in your comfortable footwear.
Slim Foot Warmers are designed to fit in tight spaces with less room, such as snug shoes, ski boots and skates.
Thick Foot Warmers are the ultimate in warmth. They take a just a tiny bit more room, a little extra space in your footwear (not much) for their comfortable fit.