Tank bags come in a wide variety of sizes and styles. Some are just big enough for a pair of gloves and a roll of lifesavers, others are big enough to go camping.
I'm not sure if the Ninja 250 has a steel tank, but if so you could use a magnetic tank bag. I have one, an Eclipse, and I love it. It's expandable, so it can be small or larger. That's a nice feature because usually it's not full.
Google 'motorcycle tank bag' and you'll see the variety that is available. The tank on a Ninja is pretty big (for such a small bike) so probably any of them would work. Get the measurements on the bag, then go measure your tank and see how it would fit. The height probably doesn't matter as much, because you're sitting mostly upright on the bike, but you can see how much room you have if you sit on the bike with a ruler or tape measure.