I picked up this pack for some fast packing last summer and have not used it since. It has a 40L carrying capacity but also folds down enough for day runs/hikes or similar outings where a tiny running vest just doesn't cut it. It also features plenty of storage on the shoulder straps in addition to the flask holders, and has two waterproof compartments (one on the shoulder strap and another on the main body).
Designed to fit 17-19 inch torso length. However, because it lacks riser adjustment straps, I prefer to go a size down in these at around 19-20 inch torso size. This allows the weight to sit a bit higher on my back.
Message with any questions.