wide but not too wide
I'm running 710mm bars on my 29er and I like the width, but I think I'm going to trim then down to about 680mm. For some of the places I ride the 710mm is a tad wide. I'd like a little more breathing room between my nuckles and the trees.
I don't think there is a hard and fast rule about stem sizing, but technically speaking as you go wider you are increasing your reach, so to keep the reach the same with wider bars you need to use a shorter stem or reduce your drop to keep the reach the same. Like wise, as you go narrower you are decreasing your reach, so to equalize you can go longer in the stem or increase your drop.
I think two important factors for bar width selection are:
1.) your size. if you are 5'6", a really wide bar might not make sense.
2.) your usual terrain. if you ride a lot of technical tree buzzing single track then really wide might not make sense.
I think for most trails anything up to 710mm will not be a problem. wider than that and you might be asking to punch a few trees. although you might be able to take flight😉
just my 2 cents.