"For a day hike, this is perfect for women."
A 2L hydration pack will fit it in snugly. I have an Osprey and a Platypus @L hydration pack. The Osprey rigid structure wont fit in and the company's smallest hydration pack is a 2L. Platypus offers smaller hydration packs but the 2L will fit just fine.
"Perfect pack for commuting cycling and day hiking"
Purchased this pack for city commuting and weekend outings. The hydration sleeve holds my iPad plus hydration pack when cycling. The fabric is durable and rather elegant. There is an internal security pocket plus an outer pocket for my train ticket, etc. The main compartment holds notebooks, lunch, etc. The shoulder straps are very comfortable - perfect for a smaller torso. The sturnam strap adds a measure of security. The top handle is very study - ditto zippers and pulls. I am impressed with the design of this small sleek pack which isn't really all that small. btw - measurement flat is 17x12 inches.