Shoe sizing, especially online, is not a perfect process. That's why we have a free exchange policy.
We have sizing instructions at the bottom of the product description for the Hana.
The Hana fits "true-to-size" or a TINY bit small. So you'll probably be okay selecting your "usual" shoe size. If you think you're "between sizes," go up a 1/2 size.
Also, the Hana is wider than conventional shoes. The canvas upper can stretch to become a bit wider.
Keep in mind, though, that because the foot is three-dimensional, any two-dimensional measuring tool, such as a ruler or Brannock device, can only approximate your true shoe size. Manufacturers model their shoes on different foot shapes, which is why you might be an 8 in one brand and a 9 in another.
Sizing for a shoe is different than a sandal, so if you own one of our sandals, that doesn't mean you'll pick the same size in the Hana.
If your foot is really narrow, you may want to check out the Lena.