Size Small/Medium. Chest measurement: 34-38” Lululemon running vest with tons of features. Space for a water bottle, zipper waterproof pocket for your phone. Lots of pockets for gels, snacks, and hydration. It was worn only for one run, I’m selling because it’s too large for me. Slots on the back of the vest for hiking poles. Camelback bladder can fit in pocket behind the neck/back. Emergency whistle included