I believe the size is free size. It can be adjusted around the armpits and chest. Flask is not included, only the main body shown in the photos. I purchased this on this site six months ago and used it about twice. I'm not planning to use it for now, so I'm selling it. The print on the back is peeling off. Please check the condition in the photos. Lightweight mesh material Salomon backpack, comfortable fit. - Brand: Salomon - Color: Dark Green - Features: Mesh material, lightweight design Thank you for looking.