Mine seem fairly true to size, but they are the newports, not the slides (and mens sized, if that matters). The newports have a little "toe" indentation in them, so I can tell where they expect the toes to go, and it seems right when I'm standing or walking. Maybe the slides are similar?

REI is usually pretty good about taking back things ordered online. Maybe take them to the store and try on a size down? Unless they don't have them in the store

I always find that slides fit funny, and sandals in general even. The Keen slides do seem a little... weirder than normal ones, but I've only worn wrong-sized ones (someone I know has them).