I've always read to measure across the back from acromion to acromion (the point at the outside top of your shoulderblades).

I've been measured at exactly 38 cm, but I can't comfortably ride bars narrower than 40 cm. How I knew is that when I started to fatigue on a long ride on the 38s that came on my bike, while riding on the hoods, my wrists would break to the outside. I'm very comfortable on 42 cm bars as well, just not as aerodynamic. Years ago I actually built up my race bike with 42s, they're that much more comfortable for me.

Some of it will depend on exactly where the manufacturer measures the bars, too. Center-to-center, outside-to-outside, hooks or bar ends? One company's 40 cm bars aren't going to feel the same width as another's.