When I first started riding I was on 42cm bars.

Last year I got Zipp aero bars and dropped down to 40 cm. Didn't really notice much of a difference.

This year I went to 38 cm Zipp (SL-70?) bars and at first they felt too narrow but I've gotten used to them - no pain anywhere or anything like that.

Once thing to keep in mind about these bars is they flare out a bit in the drops - so they might even be 36cm at the tops, but 38cm in the drops.

Narrow is aero!

(I'm 5'10" for reference)