when i got fitted, the guy held the bars up to the top/back of my shoulders behind my neck...so i think it's the width of the tops of your shoulders in line with your arms. which makes sense, because that's about the width of your hands if you reach your arms out straight.

you really just have to ride on them and see how it feels, though. my old bike had 44 cm bars. never thought much one way or another until i got a new bike that theoretically had 42 cm bars. it felt so much better and i realized that the 44s had been too wide. but it still wasn't perfect...and when i got fitted, we realized the "42" bars were really more like 40, or maybe 42 measured from end to end. my current bars are 42 cm center to center and they're perfect.

what do you have now? how does it feel?