These are descisions you are going to have to make for yourself.
However, some input from a biased source (aka me).
Learning on a bike bigger than 250 is unnessicary, and is going to be less forgiving. The 250s are very forgiving bikes that rarely scare you. On a bigger bike, more of your limited amount of concentration will be spent on the bike, and less on riding and the other road users.
Anyone who tells you a 250 is too slow, won’t be fun, etc, has either not ridden one, or not pushed it anywhere near as hard as it can go.
A 250 cruiser, a Ninja 250, a dual sport 250. all will serve you well, the choice is simply which style you find more interesting and comfortable.
As a first vehicle, I personally am weary of motorcycles as the first vehicle. There is a lot going on as a new driver. There is even more going on as a new rider. Other road users (drivers, pedestrians, cyclists, and yes, motorcyclists) do some pretty stupid things that endanger you, in a car or on a bike. Learning to drive a car first (and putting a few years on it) gets you used to reading road trafic. You won’t be surprised by the cabby with his left turn signal flashing that immediately streaks right across three lanes of traffic, you’ll have seen it coming.
My father taught me to drive a big Explorer (1994). Sight lines sucked, you had no idea where the back end was, brakes were terrible, steering was totally numb, and the V6 made enough power to break the rear tires loose with any amount of jealousness on the gas pedal. It was a hard vehicle to drive. But it really made me work and be a better driver. You had to plan, you had to be smooth, and you had to be very aware of your surroundings.
I wasn’t allowed to drive the family sedan (a sporty little ’89 mazda 626) for over a year. By the time I was allowed to drive the little car I was pretty good driver. But I still had two accidents in my first three years of driving (hydroplaning in the slush and snow, and making a left hand turn without paying sufficient attention). I am very glad I made those mistakes in a cage.
My 2 cents