AM or trail bars are more commonly 25-26 inches...but some will occasionally run 27-28 inch bars in some cases where the terrain allows. Sometimes your physical size will dictate what width bar you like, sometimes the terrain will influence bar width (trees, etc.), and sometimes people just like 'em short, wide, or in between. On the other hand, I bet you could go to the trailhead of a really popular trail and poll people on whether they know what bar width they are running at this very moment on the bike they are riding...and I bet you'd get lots of folks who don't have the slightest idea without measuring...LOL! I generally run 26".
I am 6'2" with fairly wide shoulders. I was using a 640mm bar for the first year. I never really felt comfortable on it. For Christmas I got a 685mm (27") bar and absolutely love it. 45mm difference (almost 2") between the two but the riding is sooo much better. It it more comfortable and I have more control climbing and descending.
Bar width is purely personal preference. The easiest way to find out what you prefer is to relocate your hand positions while riding. If the narrower grip feels more comfortable, try to determine how much needs to be removed from both ends to achieve that width (don't remove more than 1/4" at a time though). If you find that you prefer a wider bar, oh well. Sell the one you got on ebay and look for a 685mm or possibly a 710mm.
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