Right bar width is easy to find. Just slide your grip inside just little bit and do a test drive. Keep sliding inside until you feel comfortable with it. Do a mark with a pen and then cut off the rest of the bar.

If its a stock bar i do recommend to buy a new one and grips to it. And buy little wider than you think about so you can start to cut down to preferred widht. Ofc if you know what width you'd like you can buy one if you find right width.

Not sure if all bars can be cut. But there is at least a few with cutting marks.