I used to work with Harley, and I'm officially 'over' the whole HOG mystic. It's become a bunch of cheese-eating Nancy boys runnin' the show these days. You should go check out a Gold Wing. It's not the cc's as much as the $$ that matter. If it has to be a 'sport bike' then Kawasaki's are great! Is it for you?
I would say you go for the 250cc motorcycle for beginners. 250cc motorcycles are not only cheap to buy and run but also you will be able to learn fast when it comes to cycling it. With a 250cc, you do not need to worry about throttle or pulling a power wheelie.
I've been riding most of my life and I would say it would depend on how tall you and how much you weigh. If you weigh 300 pounds and 6 foot 3 you're not gonna want a 250cc. You want a bike that will carry you at interstate speeds without overworking the bike. If you're tall you don't want your knees in your chest, but you need to be able to place both feet FLAT on the ground. Size of the motor is not all that important as far as it being to much motor. You can get killed just as easy on a 250 as you can on a 1500. Just when you first start riding, don't try to act like you know what you're doing. The road has it's way of humbling you. And always ride like you're invisible.