If you are interested in ultralight, the Crestrail 70 probably isn't what you are looking for. You might want to read a little more here before making that purchase.
That aside, I would get your wife involved in the selection. Fitting a pack is like fitting shoes, with many variables, physical *and* perceptual. As to how much she will carry, only experience will tell, and how much she is interested and dedicated in backpacking. I guess you could look at gear lists from the female members here, but you will be looking at the results of years of experience and a keen interest in developing an UL kit. There are so many body types, ages, physical conditioning and the rest, that makes it a very general question.
If you are introducing anyone to hiking and backpacking, I would start with day hikes and graduate to simple overnight trips. People who are not used to the wilderness have a lot to take in at once: the physical exertion, bugs, fear of getting lost, wild animals, spending your first night on the ground in absolute darkness, and so on. Some are fascinated with the process, easy-going and flexible; others may be resistant to the idea and a "hard sell." Tread lightly!