I would definitely go up one size only because they don't stretch but I really love them.  I have the same shoe that I bought years ago and they didn't have the support these have.
I got a size 10 and they fit great but those bunions from my younger years are just a little too tight but I'd say 10 is a perfect 10 without the stretch.
I'm getting 11 and giving these to my daughter who is about as n 8 1/2  and they work well enough for her.
Nice summer sandle!