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 Cassie.
The Cassie 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 Cassie is wider than conventional shoes. Some people have experienced stretching in the canvas upper to slightly 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.