To determine the AGO size that will fit you best, use a similar garment you already own that fits you well. Ensure any button or zipper closures are done up, lay your garment flat with the front facing down, and measure as follows:
Width: Measure from side seam to side seam approximately 2" below the armpit.
Length: Measure from the bottom of the collar to the edge of the bottom hem.
Underarm Length: Measure along the underside of one arm from the armpit to the wrist seam.
Compare the measurements taken to the chart below to determine your appropriate AGO size.