When I was really into MTB back in the 90s, I used bars as narrow as 53cm, and I am 2m tall! Never seemed like a stability issue even on crazy downhill and technical single track.

So when I bought a 29r a few years ago for fire road riding, and it had 80cm bars, it felt absolutely ridiculous. How do you even fit on the trail? But had to give it a chance, since that was the thing. So only cut 5cm off at a time, starting the minute I got home, giving it 100 miles between chops.

Wound up at 62cm. Still feels really wide, but somehow proportional to the hugeness of a 29r.