Here in Chicago, the Motorcycle Safety Classes fill up fast so I am currently in the middle of training three people to get there license - a guy I met in a Motorcycle Online forum (22 yr old), my wife (30-something !), and my dad (62 - today !).
I would highly recommend taking an MSF class before deciding on a bike to buy. That way, you can drop someone else's bike a few times. We are using a used Ninja 500 for training and it has been dropped 5 times: guy 3, wife 0, dad 2. I got the guy and wife already riding on the street, but my dad is not there yet.
My wife is 5' 5 and weighs 150 lbs (had a baby 4 months ago - not down to her weight yet !). She likes the 500 a lot, but I can already tell that she is going to outgrow it - a light 600 (ZX6(R)) is probably her bike.
I would recommend a used Ninja 500 or Suzuki GS500. The fairing on the Ninja can be damaged quite a bit from a slight tip over.
Get a year of riding under your belt, then decide.