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 to the hem in the front and back. 
Small: Bust is 22.5" (45" around), Front  Length: 24.5", Back Length is 25.5"
Large: Bust is 25" (50" around), Front Length: 24.5", Back Length is 25.5"
X-Large: Bust is 26.5" (53" around), Front Length: 26.5", Back Length is 27.5"