I'm a size 8, average width foot -- the Large size works for me. Yes, more than a tad tight when I first put them on, but having read the reviews, I expected that. The first morning after they arrived, I showered with them on and then wore them wet all day at work -- not exactly business-casual wear, lol, but I was on a mission and I got away with it. :) As they dried, I could actually feel the straps loosening up and the footbed starting to mold to my feet.
I did end up with a couple of rubbed spots on my feet, but with some judicious bandaid application, I was good to go a couple of days later and wore them all day walking around a dog show. The padding in the sole and the arch support is fabulous; I have somewhat controlled plantar fasciitis, i.e., I have to be really careful about my footwear. A day spent in cheap flat flip-flops usually leaves me pretty sore but not with these. I'm now finishing up the break-in process and I can tell these will be my go-to flops next summer and trusted companions on upcoming vacation trips to Mexico this winter. Really glad I found Rainbows!