I always cut down my bars; one size does NOT fit all. Standard width is 32", I run 29" in the woods and 30" for motocross.
If you get down and do pushups to failure, allowing yourself to adjust your hand placement for maximum strength, you will find the width that works best for you. For small guys like me it will always be less than 32".
The benefits in the woods are obvious but you would still be amazed at how much difference even a half-inch makes when at speed through the trees.
https://vimeo.com/163260691For motocross it's mostly just a matter of comfort and strength, but I also like a little more "breathing room" on the start line.