The size is free. The outer black part, from heel to toe, is 28cm, but it won't slip off or move around even if you're wearing socks. By the way, my foot size is 23.5cm. Washed and disinfected. I bought it 2-3 years ago, but I don't get cold easily, so I didn't use it much. I think it will be useful when it gets cold from now on.