1. Slide your brake levers, shifters, bar ends and anything else that you have on your handlebars inward as far as you can. Make sure that everything still works because sometimes shifters bind up if they are clamped on a curve. That's as much as it's functionally possible to narrow your bars.
2. You can always trim a little off and, if you still don't like it, cut off a little more. Practically, I've found that 3/4" has been about the max for me which still leaves them significantly wider than my road bars. My road bikes have bars that measure 18 1/2" to 19" wide outside-to-outside. My mountain and flat bar beaters measure 21 1/2" to 22".