I bought this sandal and like it very much. I am an 8-1/2-9 and I got the size 39, even though I should have gotten the 40 if I had followed the Wolky size chart.
If you have a medium-to-narrow foot, I would buy the smaller size, 37. Both the ankle strap and the strap across the toe are adjustable since both close with Velcro. If your foot is wide, I would get the larger size, 38. I hope this is helpful.
And if the size you order is wrong, it's not a big deal - STP will make an exchange. I hope this is helpful.