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?