Supposedly, around the width of your shoulders, although one site seemed to indicate to use what would fit inbetween your armpits, while another was almost the exterior width of your shoulders. I presume that the proper answer is inbetween those two measurements, give or take, depending upon the riders preferences? I ask as the drop bars on my bike measure 38.5cm center to center; yesterday, at a bike shop a set of 44's fit into my armpits. I presume I'd be much happier with 44's than 38's, no?

The method I used was measuring from shoulder point to shoulder point then adding 2 cm for bars measured center to center.

Using that method I should be in a 44 but the bars on my bikes range from 42-46 and I can not say that I have a strong preference between them - I am comfortable on all of them. Not sure I could adapt to a 38 though.

Hmm, I think I'm 46 or 48 across the shoulders, hard to measure on myself and I'm not sure where it's actually measured from (not that it's an exact science). I'm trying to read up a bit more, as I am thinking about a new bike for spring.