Handlebar width is related to shoulder width and depends on the handlebar type, top tube length, stem length, and position desired. Also, BITD we did "narrowest gap in trail" for bar width. The shoulder width rule of thumb is for drop bars.

Risers are generally wider with more sweep than flats, which are wider than drops for the same rider. Modern bikes use shorter stems with longer top tubes, so older bikes do better with narrow bars and vice-versa.

Lately there is a trend in bars to be wider and wider every year. For modern risers, I like to use the "most efficient pushup distance" to determine bar width. I do push ups at different hand distances and measure the outside of my palms at the position where it feels easiest to push up.

You may have to adjust your stem once you find your right bar width, the two are closely connected.