I know guys that start out with much bigger bikes than 250's and they are about your size. They never had any issue. Remember a bike, any bike only goes as fast as you turn the throttle. No different than a car. If I were you, I would find a 600 I liked and sit on it. If you can lean side to side and it doesnt feel like it is going to fall and you couldnt stop it, then thats the size I would get. I would maybe start my wife on a 250 but I would have no issue with her on a 600. Theres not that much difference in the bikes themselves.