Measuring the boys is much easier than the girls.  I am showing a girl in the photo, but you want to do the same thing and include their extra equipment in the measurement.  For sighthounds, the pants are wide enough to cover their long sleek waist for less seepage.  

If you need a certain width, put a notation in the comments section or notes section when ordering.  

XXS - Waist is 9 - 11 inches; width is 5 inches.

XS - Waist is 12 - 14 inches; width is 5 1/4 inches.

Small - Waist is 15 - 17 inches; width is 5 1/2 inches.

Medium - Waist is 19 - 22 inches; width is 7 1/4 inches.

Large - Waist is 22 - 25 inches; width is 7 1/2 inches.

XL - Waist is 30+ inches; width is 7 1/2 inches.