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 down to the hem. 
 
 Small:  Bust is  22.5" (45" around);  Length is 28"
Medium: Bust is 23.5" (47" around); Length is 28.5"
Large: Bust is 24.5" (49" around);  Length is 29"
X-Large: Bust is 25.5" (51" around); Length is 30"