As with all rules, individuals invite exceptions. Traditionally, bar width (center to center) should match that bony bump at the end of your shoulder. I've gone wider over the years. My first serious bike (21 inches 24 years ago) came with 38cm bars. (More aero is better?)
I'm 5'8" and fairly average (medium shirt size) and my bar width has crept up thru 42 to currently 44cm which I find works very well. I even have a 46cm on one bike......which also works well for climbing and general riding. 42 would feel noticably narrow to me now.
One reason for the creep up in bar width was the idea that it "opens" up your chest for better breathing. I tend to think there's something to that from my experience gasping up some hills.