Question about bar fit for my wife.. is it normal practice to cut down handlebars to width? What is the proper measurement? In road biking, one typically uses the width of their shoulders as a starting point, but clearly with Mtb, its much wider than that. My wife uses a 38-40cm road bar, if that's any indication of things.

Thanks!

i would say mtb guidelines for bar width often have more to do with riding style, terrain, general preference, and lastly, ergonomics. singlespeeders and gear mashers tend to prefer wider bars, as it taxes the upper body less when standing (arms have more leverage to brace against pedalling forces). wider bars lower steering effort, but require the hands to move farther for a given steering input. some areas have trees that seem to grow exactly 25" apart, so a 24" bar works pretty well there.
all that said, it's very rare to see a bar narrower than 24" these days (except on my bike, im old school that way). i suggest sliding the controls and grips in on her current bar to around 24 or 25" and see how she likes it. cut them later.

Well said!

Depends what bike you're riding. Here in the UK, race guys use much less than 640mm - esp. if they are flat 'bars. Trail 'bars are anything from 680mm to 750mm. The UK mags reckon on 715mm min, but they can't ride the tight trees I ride. I have mine at 660mm, wouldn't want any more. I used to cut my 660mms down, but not any more. 660mm suits me on my 26" bikes. Cutting down carbon 'bars or seatposts won't save you may grams anyway.

My 29er has 720mm 'bars, but that makes sense given the extra leverage needed to steer (and hang on if a HT). 29ers haven't taken over here in the UK, although the big companies are trying to force us to ride them !!

It's very personal, I run my XC hard tail at 600mm ... anything wider just feels wrong to me. I find it really weird switching from mates tail bikes with wider bars, or even other XC bikes with wide bars, and they are probably only at 660ish ... how does anyone get around tight trails with 700+ bars ? I can only guess that each person's terrain is really different.