Bars are available in a range of widths and when a choice is available the majority of cyclists choose a width that is equal to the width of their shoulder line (the distance between the shoulder joints). A wider bar opens the chest for better breathing and greater leverage whereas a narrower bar is more aerodynamic.
It is extremely important to be aware that manufacturers measure their bars differently. Some measure centre to centre, others measure outside to outside. Manufacturers that measure outside to outside include Deda Elementi. Manufacturers that measure centre to centre include Cinelli, Easton, ITM, Pro, Profile, Prolite and Ritchey.
As a general rule you should consider the following size guide. It should be noted however that over 85% of experienced male cyclists choose bars with a width of 44cm (outside to outside) and the majority of lady cyclists select a 38cm or 40cm bar (outside to outside).