I got a pair for free so I can perfectly help you. I'm a size US 10 with narrow feet. I was given a size M. They fit, even with socks; here's the kicker, they could definitely use more room. When it comes to walking fast, or 'sandal-sprinting' or walking in the rain, that extra room would help. So let's say if you're a size 7.5 stick with a small. If you're a size 10.5 go with a Large. If you're 11.5+ go with XL
For me I kinda just got a soft bristle brush and scrubbed it gently with a little bit of soap. Or you could buy one of those magic erasers It might work as well.