The wider your bars, the more you'll be braced with your arms. That means your shoulders, arms, elbows, wrists and hands will be taking the brunt of all the forces you encounter on the road.

The common method of measuring bars for a road bike has been to match the measurement between the acromion process of both shoulders. That had me on 42 cm bars. But I found out that I like narrower bars and went to 38cm bars. It seems to make the bike more natural feeling for handling twisty downhill bends. As well it's easier to make last minute darts to the side to miss road hazards that suddenly appear out of nowhere.

But the big thing for bars is not any one magical formula. It's simply which do you prefer.


edit.... Oh yeah, the other big thing that going to the narrower bars did for me was to keep my wrists straight when in the drops for a while. With the 42cm bars, my wrists would tend to bend when in the drops and that made for numb hands.