I would disagree about the 18... that's a BIG pack for how much it holds. I've owned two of them, and they're decent sized.
If you want small, look for a freeride ABS. The bag setup on the BCA makes the top of it big & rigid by design, making it less comfortable for a smaller rider. The freeride was the first pack My wife (5'1") felt comfortable with. The other advantage of it is that it's usually easy to find used.