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)