As others have already said. I'd recommend taking the MSF course first, buying your gear, and then buying a used Ninja 250 or 500.
Tony

Personally, I think the "Buy a bike that FITS you" philosophy is overrated. It's easier to make a GOOD bike (that you like) FIT YOU than it is to make a crappy bike that FITS YOU into a good bike. A different seat, lowering link, handlebars, windshield, etc, can go a long way to making your dream bike fit you.

I bought a 2010 Z1000 as my first bike... I figured, why have experience on various bikes before moving up? If I rode the same bike all along, having more experience on it would be better... If I was expecting it to handle how my previous bike did wouldn't that throw me off?

I took the MSF course and at 6'5" realized a 250 would never work for me. I ended up getting a used ZR7. Great bike, just big enough for my lanky frame, and easy to ride.

If you take the MSF course you'll realize right away what you should start out with. Some folks pick up everything really well while others struggle. If you have issues working the 250s in the class, don't buy anything bigger.

A lot of good advice here.

I come from a land where small displacement bikes prevail and I'm self taught. My first 3 bikes were 125 enduros (back in the mid-70's) and I had a blast with them and knowing me I would have been hurt on anything bigger. On the flip side of the coin I have a friend who got an R1 as his first bike and after surviving his first scare he settled down, learned patience and loves that bike to this day so it can be done but there's risk the bigger you go.

The last thing you need to worry about is the 250 handling the interstate, I've never owned the baby Ninja but I did put over 100,000 miles on the weakest 250 on the planet (Honda Nighthawk) once covering 920 miles of interstate in 17 hours. I now own a Hyosung GT250 with 93,000 miles on the clock and that one I've toured the lower 48 states with fully loaded. The Hyosung has twice the HP of the Nighthawk and comes with a full size frame, I've also led groups of liter class bikes in the mountains with no problems (some of those from this very site)and to this day averages 70 mpg doing it.

These days I commute with the 250 with occasional weekend rides but save the long distance hauls for my Triumph Bonneville.

For your application I would recommend the 250 mostly for the cost involved and I'm not talking about the bike but the maintenance and insurance that will follow. Contrary to popular belief a 250 can easily be a long term mount and not just a parking lot bike or a stepping stone to a 'real' bike.

"Mo, is not always mo better"

I started on a 250cc bike. I moved up to a CBR 600F3. That's a four-cylinder supersport. I moved to the 2013 Ninja 650 and it fits me the best all around. It took me less than a month to want something bigger. To be honest I felt the 250 lacked passing power on the freeway, and the fun factor wasn't as high at all. You'll outgrow the 250 and end up selling it, or just not riding it as much. If you can respect the 650 you'll learn just as quickly with it.