I think you'll be ok with getting a 600 class bike but I would not get a brand new one. I think you might get bored on a 250 super quick. And who says you have to take the 600 to the max. Just make sure you don't dick around, show some restraint. But get one a few years old. Because I guarantee that you WILL either crash, tip the bike over, get it knocked over somehow, forget to put your stand down, or not maintain it correctly, ect...
Your first bike is the bike that you learn on, not just how to ride but also how to take care of it, how to service it, ect.. so you kinda want a bike that is not soo much $$$.
Also DO NOT even buy a bike without taking at least taking the beginners motorcycle course. Its usually a 2 or 3 day class and you get to use their bike (so if you crash, who cares!) You also get your license from this class.
Thank God for Guardian Angels... Especially This One!