A bar should be as wide as your shoulder.
Wider bars open your chest cavity more, thus lungs could expand larger, more air into your lungs.
Narrow bars are good for weaving through traffic, slightly more aerodynamic.
My bars are 18"/457mm on my commuter and 18.5"/470mm on my Gutterball. I cut a lot more than 10mm off mine ;-) Bar width is a personal preference. Start wide and ride with your hands in different positions till you find a width you like. Then cut.