Here are a few illustrations on how to measure for pants to fit girls.  

First, measure around the smallest part of the waist.  When ordering the correct size, keep in mind that there is anywhere from 2 inches to 5 inches of velcro for adjusting to fit.  As each size increases, there is more velcro to work with and adjust.  

Second, measure for the tail set.  Measure from waistline to tail.  Please let me know in the comments section or the notes section when checking out whether you need more room here.

The sizes are as follows:  

XS - Waist is 10-12 inches; Tail set - 3 1/4 inches.

Small - Waist is 14-16 inches; Tail set - 6 inches.

Medium - Waist is 17-20 inches; Tail set - 7 inches.

Large - Waist is 22-26 inches; Tail set - 8 1/2 inches.

XL - Waist is 26-29 inches; Tail set 9 1/4 inches.  (There is also a size not listed that I call XL + 4 which is an additional 4 inches for tailset of 13 1/4 inches)

XXL - Waist is 28+; Tail set 11 1/4 inches.  (Great Dane size).