Bars keep getting wider -- and that's often a good thing. But with gravel bars that flare at the drops, the brake hoods tend to point inward more, which can have a narrowing effect.

So it all depends on what kind of bars and what kind of riding you do -- my shoulder-to shoulder width is about 17.5", and I have a 29" MTB bar, a 42cm road bar, a 700 mm "alt" bar, and a gravel bar that's 42 cm at the hoods but flares out to 50 cm at the ends.

If you're just looking at regular road bars, I'd start with a 42. For anything else, it's probably going to take some trial and error.