The waist is measured from side seam to side seam across the front of the waistband.  Double to get the circumference.  It is then measured across the waist band when stretched out ( double to get the circumference).  The hips are measured across the front at the pockets from side seam to side seam (double to get the circumference).  The length is measured from the waistband straight down to the hem.  The inseam is  measured from the crotch seam straight down to the hem.  
X-Small: Waist is 13" (26" around) but will stretch to 20" (40" around), hips are 25", length is 32", inseam is 23"
L/XL: Waist is 15" (30" around) but will stretch to 23" (46" around), hips are 26", length is 33.5", inseam is 23"