The Adventure Vest 6.0 uses the roomier Vest construction, with a wider, taller back and spacious arm openings. Ultimate Direction sizes this generation around the lower ribs.
Small: Lower ribs 58–81cm / 23–32in
Medium: Lower ribs 76–99cm / 30–39in
Large: Lower ribs 94–117cm / 37–46in
Measure around the lower ribs while wearing the clothing you intend to run in. A loaded vest will fit differently from an empty one, so final adjustment should be made with the hydration and equipment you expect to carry.