When I was researching an '80s Italian bike, I saw in some Cinelli catalogues of that era that the drop bars were only available in 38, 40, and 42 cm.

I'm currently in the process of replacing my stock 44 cm drop bars with 40 cm aero drop bars. One thing I've noticed is that both bars (Cannondale originals and Bontranger replacements) and are clearly measured c/c from the hoods. Both are about 2 cm wider at the bar ends. I've seen a few people suggest that handlebars are usually measured at the drops, but that doesn't seem to be the case with these two modern bars.