There is a Ninja 250, 500, 600, 750, 1000 etc.. The higher the number the more powerful the motor. Do not get anything less than the 500 and even that is pretty weak on power.
The Ninja 250 is a glorified scooter with a fairing on it. The ninja 500 is a bit better but as you look at their tires they are very skinny and not good for freeway riding. See video of bike below