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.
I got a great berghaus arÍte extreme from the berghaus outlet for £30 which fits lovely . The other newer ones that I found fit nice were ospreys womens range and the Lowe alpine. Bit pricier but I'm sure you could find a bargain somewhere