Do you have just a crappy old razor or something similar with adjustable size bars?
To get the perfect fit, what I reccomend is to get the adjustable bars, while still on the scooter adjust them to right under your groin area. Once you get them to that height, take them off and measure them(in inches of course). Say they were 22 inches and a half high, round them to 22.
This is what I did and my bars are PERFECT for me.