GoLite makes the Jam in both men's and women's models. It is not unisex. Since you're shopping for your daughter to use as an overnight pack, I would definitely buy her the women's model. There is a difference for the size, alignment and location of straps and waiste belt for women's packs - better for your daughter to comfortably carry. Yet, for you to use it as a day pack, it would be fine.
I think the volumes quoted here were for the men's packs, not the women's. I own both a women's Pinnacle (50L) and Jam (~38L), both in size medium. Both are nice comfortable packs. I've been using the Pinnacle for over a year now, including several week trips and a CT thru-hike last summer. It held up well and carried all loads comfortably. I just got the Jam so I haven't used it overnight yet, but it is very similar to the Pinnacle, just smaller.
Checkout GoLite's website for complete specs to match your daughter's age, height, weight, torso, shoulder and waist measurements and body-type, etc, but I suspect the Jam medium would be fine for her. For reference, I am 5'-2",108 lbs.