Handlebars are generally measured against a person's shoulder width. Conventional bike fitting practice suggests having the handlebars, or at least the primary gripping surface, at about the same width as your shoulders, so your arms are straight out in front of you. Some prefer narrower than this and some prefer wider than this. I have relatively broad shoulders, and I prefer bars that are even slightly wider than that, and 660-720mm handlebars tend to work well for me. I have some that are in the 580mm range, and they work, but I feel a little tight on them.

Depending on the style of bar end you're getting, a bar end can work well out at the ends of wide bars. I have some that go on the end of the bar like caps, so they sort of look like T-handles. These work great at the end of wider bars as grips for standing up and climbing. I have these on my mountain bike, which has a 680mm bar if I recall correctly. My Giant Roam has a slightly shorter 660mm bar, and I have bar ends on it that come forward and then curve in. I have these wrapped with foam gripping surface and they work well to augment the main grips. I use these much like someone on drop bars would ride on the hoods of the brake levers. These probably wouldn't be quite as comfortable at the end of wider bars, at least the way I use them.

I recommend experimenting some to see what you find comfortable. If you like your current bar, then you probably don't need to change or cut it just because you're adding bar ends. You will probably need to slide your grips inward just a bit to allow the bar end to mount. Try it like that for a while and see if you find it comfortable. If you think it's too wide, then slide everything in 1/4" or so. And keep doing that until you find the right width. Even small measurements on the bar can make big differences in how everything feels so adjust in smaller increments until you find the right adjustment. After you're sure you've found the right adjustment, then you could consider cutting your bar to length, or replacing it with one the length you want.