In general, we recommend going with something a bit wider than you might normally choose for your road bike.
Another way to get a rough starting point is to look at your bike frame size. If you are riding a XS or S sized frame, the 44cm could be a good match. If your frame is in the Medium size ranges, then the 47cm width would be a good choice. And finally, if you are riding a Large or XL frame the 50cm width could be a good fit. And if you are between sizes or can't decide, we always suggest going for the larger size.
Particularly when riding off-road (but on road as well), the additional leverage and stability afforded by wider handlebars dramatically improve handling and control, especially while descending and climbing. An interesting parallel can be made with mountain bike handlebars, as an example - 76-80cm wide handlebars are now common even though almost nobody has shoulders that wide and it was not until a few years ago that people started realizing that wider was really helpful.
Often times, people are concerned about getting a bar that is wider than their shoulder width but in reality the difference in effective arm angle is negligible between width changes of a few cms, but the effect on control and comfort is dramatic. Very few people want to go narrower once they've tried a wider bar.
Basically, you can't go wrong but we are recommending that wider can be better. And keep in mind, if you do try the bar and end up deciding it's not for you, know that the KSHB is covered by our 360-Degree Guarantee, so you can ride it for 3 months and exchange it for another size if it is not working out.