I was wondering how to determine the best width for flat bars for a touring bike? I know when I built up a road bike , with drop bars, last year the dimension was based on shoulder width.
Advice?
Thanks!

if in doubt, get them longer than you think you need. then cut them off it they don't work for you.

Stand up, close your eyes, hold your hands out and grip imaginary flat bar. Measure width, start there. I like bar ends too for a change of position.

Now I favor Trekking Bars ,, there, you have a bunch of hand holds,

though the width of the bar itself is not changed..

and All the flat bar controls Fit.. Mine It's a gripshift + hydraulic brakes

This a Tandem?

There's a point where your arms will feel as if they're relaxed and "floating". Further in or out will cause tension in arms or shoulders.