You are a little short for this bike. The Ninja 250 has a seat height of 77.5 cms
For someone your size, the most important thing to consider is seat height. You have to be able to balance the bike while stopped in traffic. Best if you can sit and put both feet flat on the ground. With a seat too high, you will only be able to put one foot on the ground. You may be forced to lean the bike to the side and can easily lose your balance and drop the bike. This could be dangerous in traffic. At the very least, it will be embarrassing. This could also be a problem when you take your motorcycle license road test.
There are lowering kits available for some bikes, but these can cause problems. They change the suspension geometry and can cause handling problems. The reduced ground clearance can cause the bike to drag the pavement in corners, which can cause loss of control. The lower suspension can cause the tire to rub the fender. The kick stand will need to be altered. An ill handling lower sport bike is kind of pointless, don't you think? You can also lower the seat height a little bit by shaving some of the cushion from the seat. However, this can cause comfort issues, especially on longer rides.
You may be better off with a small cruiser instead.
The Suzuki Boulevard S40 is considered a Cruiser. This bike has a seat height of 70 cms. It has a 650CC air cooled single cylinder engine. It is belt driven for less maintenance.
If you are limited to 250CC try the Yamaha Star V-Star 250. It has a seat height of 68.6 cms. This is a cruiser style 250 CC air cooled V-twin engine.
I suggest going to some of the motorcycle dealers and find out for yourself. Sit on a few bikes. If you can't put both feet flat on the ground, move on to another bike. While you are at it, stay on any potential bikes for several minutes. Put the bike on the center stand if it has one. Sit in the riding position for several minutes and see how well the bike fits you and how comfortable the riding position is. An uncomfortable bike will not be very much fun to ride.