The engine size 250, 600, and 1,000 makes a huge difference!
The larger the engine size, the faster the bike will go, but it also makes the bike harder to control.
This is why beginners start with a 250, and move up as their skills improve.
A 600cc sport bike can move! I recommend a season (crash free) on a 250 before moving to a 600. There is a very big difference in power and handling.
At the level of a 1,000 Ninja, the amount of control needed to keep safe is about the same level as a jet fighter plane.
If you are wondering if you want one, I would say the Ninja 250 would be best for a beginner, a 600 Ninja is a fast bike, and requires a good bit of skill to manuever.
A 1000cc bike (especially a sport bike) is a monster! I have noticed that most people that ride a 1,000cc bike can't handle it, and it really is better for the track than the street. Sport tourers and cruisers are different, they are not set up for racing, and generally not used for high speed.
I find my best size for a bike is 600-750cc, and that will get me anywhere I want to go, as well as being a great pleasure to ride in the turns. By the way, sport bikes suck for comfort, so no cross country trips for them!
Have a great day!