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I actually had the same problem, I originally bought this herschel backpack:
but as I'm also 5', it was just too big, the bottom of the backpack sat uncomfortably on top of my tailbone.
I ended buying a second backpack from herschel:
and that size was much better for my body frame. That size still fits my lap top, book, charger, water bottle, and a light sweater without looking overwhelming.
I would definitely recommend getting the smaller size for her unless she really does need to carry a lot of things.
That being said, I love the brand, they're very well-made while still being practical. :)