The waist is measured from side seam to side seam across the front of the waistline.  Double to get the circumference.  The length is measured from the waistband straight down to the hem.  Inseam is measured from the crotch seam straight down to the hem. 
One size: Waist is 13" (26" around), but will stretch to 24.5" (49" around).  Length is 33", Inseam is 26"