I wear a size 8 (Normal width) women, and I buy the small in every style of Balega socks. They all fit perfect with no slippage into my walking or running shoes. I love Balega socks! They are the only socks that I buy now.
I wear a size 8 in women’s. I have tried both medium and small and prefer the smalls. They fit snugly, don’t slip, and I don’t get any rubbing. The tabs stay in place and protect my heels With mediums, they come up higher and are a little loose, allowing the sock to slide while walking/running.