In reply to ford23:
Best to try for comfort. Don't get stuck with just women's ones, I use several generic ones. I tighten the waist belt and chest strap so that they support the weight and the shoulder straps sit clear of the top of my shoulders, so for me a long back length is good. I also like minimal framework/ padding on them as I just adds weight. Its so much personal preference that I would be reluctant to buy one for someone else.