I'm 5'7" and learned to ride on a TTR125L starting in April of this year. I absolutely LOVED that bike and told my husband after every ride, "I think Yamaha built this bike just for me!" It was so much fun and so very forgiving of my many mistakes. It was easy to start (always 1st or 2nd kick), and I could put both feet flat on the ground; the power was smooth, and I developed a lot of trust in the bike being able to handle difficult or tricky terrain. It was great for my self-confidence.
In August, however, I felt that I was ready to move up to a bigger bike--I had ridden street bikes previously and progressed faster on a dirt bike than I anticipated. So now I'm riding a CRF230F, and I really love it. (Yes, I also think that Honda built this bike just for me!) I considered getting an even bigger bike, but my learning curve is still pretty steep, and I wanted a bike where I wouldn't have any problems putting my feet down on the side of a hill (I only do trail riding, with lots of hills, and often ride by myself).
I haven't ever regretted any of my bike choices, despite LOTS of advice that I should get bigger bikes before both of my purchases. At least for me, many of the challenges that come with riding are in my mind--i.e., if I'm thinking "Oh, I can't do that", I've already set myself up for failure. So feeling comfortable on my bike, and believing that the bike and I are working "together" (as opposed to me trying to "tame" a bike that's too much for my skill level at this point) is critical in my riding.
Everyone likes to learn in a different manner (i.e., some people like a zone of comfort, some want a tremendous challenge, and others are all along the spectrum in between). So you will get a lot of different opinions. Good luck, and let us know what your wife ends up getting!