The Ninja 250 is a great bike. It would be perfect except possibly for two things.
1. If you were planning to go fairly long distances at high speed, like, say, 15 miles to work on the freeway. A 250 will go 70 mph, but on a long trip at that speed it is beating its little heart out. It's not as stable, not as comfortable, not as safe. In that case I'd go for a 500. A 600 is not too big.
2. If you sit on one, at your size, you might find it a little cramped. You can learn a lot by just sitting on bikes. Put your hands on the handlebars, your feet on the pegs, then ask yourself 'Could I sit like this for 2 hours?'
My first bike, I got it to go to school and back, but I loved it so much I ended up going for long trips on weekends, maybe 100-150 miles in a day. So keep that in mind. 8^)
Good luck! And congratulations in advance. 8^)