Ultimate Direction sizes the Ultra Vesta 6.0 around the lower ribs. Its narrower shoulder straps and back provide a more tailored fit for smaller frames than the Ultra Vest 6.0.
Small: Lower ribs 54–91cm / 21–36in
Medium: Lower ribs 61–101cm / 24–40in
Large: Lower ribs 71–106cm / 28–42in
X-Large: Lower ribs 97–116cm / 38–46in
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.