You want to be able to sit on a bike with both feet flat on the ground. This is a problem sometimes with short people, and girls (5'4" is not short for a girl, but you know what I mean. Sometimes we say 'inseam challenged'.)
They publish bike seat heights but the only real way to tell is to sit on one.
As for the Kawi 250, it's everyone's idea of the perfect starter bike. I'm sure you'd love it. A 350 or 400 would be better, only because you could cruise on the freeway better. A 500 or even 600 is not too big, it's not really that much bigger than a 250.
You might check out the Buell Blast also. It's a 500. And like the Ninja 250, it was designed and intended for 'entry level'.