The right width is whatever is comfortable for you.

I don't know when the fashion for wide bars started but they used to be narrower in the 70's and 80's and earlier. Look at some old road bikes and you will see lots of 40cm or even 38cm handlebars.

Personally, I am comfortable on narrow bars. My favorite 80's racing bike has 40cm bars and they are perfect for me. My new bike will have 42cm bars and I'm actually worried that they will be too wide.

I don't buy the argument that wider bars let you breathe better. For one thing, my elbows articulate, I can keep my hands close with my elbows wide. For another thing, watch the pros climbing. Long climbs are exactly when they need the most aerobic capacity and yet they sit up and put their hands close together on the bar tops. And certainly wider bars are less aerodynamic, you probably lose more by moving your arms out 6cm than you gain with deep section rims and flat spokes.