What size motorcycle should my next bike be?

That depends upon what YOUR 'end goal' of riding is?  That also depends if you are over 6 foot tall and over 200 pounds? Are you physically comfortable on long rides on the 250r Ninja?  Do you enjoy the tight turns on the 250? Have you been riding threw tight turns on the 250 pulling the bike over with your outside knee? Pulling up on the outside handle bar, pushing down on the inside bar?  Have you pushed the 250 outside of it's limits and had to make recovery actions?  Have you ever been on compromised surfaces and had to make any recovery actions?

If you haven't experienced any of the above... It's a lot easier to recover any errors on the 250 than on a larger bike.  Double, triple, or quadruple the horsepower, increase the bike weight, move the center of gravity of that increased weight a greater distance from road surface, and those errors ARE going to happen more frequently and will take much more rapid and ACURATE recovery skills!  How many completely  different actions have you developed to recover from road error?

... and then there are riders that never have in their riding history pushed a sport bike into the corners of it's performance envelope... and never will.

Only you can make those decisions, only you know your end goal? Or you can thank Pearl for an answer of, "Any size you want"!  We all have to step off the porch  sometime! 

havent seen you ride. Therefore I have no idea what your skill set is at this point and neither does anyone else . Nobody can give you that answer . 

A 600 or  750 Gixxer is a few steps up from a Ninja 250.  I would suggest at least one more step between a superbike and your current ride, such as a Ninja 400, or a CBR 500

A 750 Ninja would be your next  good ride.

whatever size you want