Hakama sizes are based on a simple formula. If you know the length or your outseam, which is the distance from your belt to your ankle, then you can calculate what size hakama to get with this simple formula. Simply divide the inches by 1.49 (or the centimeters by 3.79). So, if your outseam measures 40 inches, your hakama size would be around a size 27. However, if you are buying a cotton hakama, the hakama will shrink some. So, you may want to get a hakama one size larger to compensate. Then, wash the hakama before you wear it the first time to allow it to shrink down to size. Even if you order a polyester or tetron hakama, you may want to go a size larger and then have someone tailor it for you.
Here’s a chart of hakama sizes.