The bust is measured from underarm seam to underarm seam across the front.  Double to get the circumference.  The length is measured from the shoulder seam straight down to the hem. 
Small: Bust is 20.5" (41" around), Length is 25" 
Medium: Bust is 21.5" (43" around), Length is 25.5"
Large: Bust is 22.5" (45" around), Length is 26"