I'm in close to the same boat as you are.
I am 6 foot and weigh about 220 or so. I am a fatass.
I personally went with a 600 for a first bike. It's older, so I paid cash just like you to forget about the full coverage insurance. I plan on probably buying a Ninja ZX9R when I learn to ride WELL. Probably not till next year.
Here is my take on the size issue, as I've been told on here and been told in person. If you weigh around 200, you'll want a 500 or 600 as a starter bike. The 250 will feel very sluggish to you in just a short while. I personally went with a 600 over a 500 because I got a good deal on one in good shape.
I actually asked a salesman which is better. I know normally he would say whichever bike is more expensive is better just to make more money, but I was actually looking at used bikes. The Ninja 250 I was looking at was $1400 bucks, and the Ninja 600 I was looking at was $1450. So the price difference wasn't enough for him to tell me I needed a bigger bike just for him to make more money. I asked him which he would suggest, and he said that for a guy my size, I'd be back as soon as I learned how to ride to get a bigger bike if I bought the 250. I took his word, and got the 600.
One other thing that you'll want to do is actually go sit on them. The 600 felt good to me, the 250, to me, felt like I was sitting on a moped. It just didn't feel very good to me.
Good luck, I'm sure others will have better information that I did, I'm just a newbie and figured I'd let you know what I've been told!!