Bit of trial and error.

For road drops, a good point to start off is to measure your shoulders (this guide recomends measuring the distance between the "points" on the front of your shoulders - http://www.bicycling.com/article/1,6...056-1,00.html). Can also from armpit to armpit.

This will give you a good idea of bar width. But each manufacturer seems to measure a little differently and this measurement is only a starting guide. I generally find it helpful to go to the LBS and handle a few bars to really get an idea of what seems comfortable.