I got the REI Co-op Flash Pack in the size extra small. It’s 2 pounds and 9 ounces and says it fits torso length 16 inches and waist sizes between 28-30 inches. I’m 5’3 and weigh 100 pounds and it fits me perfectly. I love the water bottle storage and the mesh front pocket to keep light things that you want to have easily accessible. When we were in the Rocky Mountains I kept my rain cover and my rain jacket in the front pocket and could easily grab them when it started raining in the afternoons.
Another thing that I love about this pack is the roll top closure. You can snug the whole pack down to whatever size you need. I had a spinal fusion surgery in 2018 and I have some nerve damage that makes my back sensitive to the touch. The mesh back and the soft padding is comfortable and doesn’t hurt my back at all. The Overall, this pack is lightweight, comfortable, durable and very easy to use!