This pack worked perfectly for me! I am 5'5" 160 lbs I bought the S/M. I used this everyday during my Yosemite trip. Although it was a little large for a day pack - I had plenty of room for water, food, and other "maybe" items (jacket, bug spray, sunscreen, flashlight) it was never uncomfortable and DID NOT CHAFE!! Another major plus was the fact that I used it as a carry on! I can also fit my trekking poles inside for checked bag trips and it has handy loops for the poles when you are out on a trail. This pack is my absolute favorite!!!