You might just be able to go into a backpacking store and grab a pack off the shelf that fits. But women have narrower shoulders and broader hips than men, so most men's packs won't fit well. Fortunately a lot of pack makers now design packs specifically for women. One very good one: The Arc'Teryx Bora 62 ($325). A nicely designed pack, with a great suspension, a sleeping bag compartment, a slot for a hydration bottle and a molded back that helps keep you cool. It comes in three sizes and my guess is the small will be the one for you-it fits torso lengths of 14 to 17 inches and has 3,500 cubic inches of room.
Admittedly, that's a little steep price-wise, although it's also a long-term investment that should last many years. Mountainsmith's Chimera 4000 in a women's model is more affordable at $260, but also not sized quite as small; it's for torso lengths of 16inches and longer, which I think will be too big. On the other hand, the Lowe Alpine Alpamayo ($260) can accommodate torsos down to 13 inches. And it's pretty big-more than 4,000 cubic inches-so if you have to carry a lot of bulky stuff, you can. L.L. Bean's Mount Washington ($149) comes in a women's model, but with a 15-inch minimum torso length it might be a touch too tall.