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 both the front and back.
This top does have dolman sleeves so the bust will appear bigger.  Top runs true to size.
X-Small: Bust is 29.5" (59" around), Front Length: 24.5", Back Length is 28.5"
Small: Bust is 30" (60" around), Front Length: 25", Back Length is 29"
Medium: Bust is 30.5" (61" around), Front Length: 26", Back Length is 30"
Large: Bust is 31.5" (63" around), Front Length: 27", Back Length is 31"
X-Large: Bust is 32" (64" around), Front Length: 27", Back Length is 31"