Sizes run true to US sizing. 

To measure hips, measure around the fullest part of the bottom, with tautness of tape at your comfort level. 

Here is a step-by-step guide with pictures for reference on measuring hips.

Hip measurements in inches:

S                   35 - 37 in.

M                  38 - 39 in.

L                   40 - 41 in.

XL                 42 - 43 in.

XXL              43 - 44 in.