For comfort, the rule of thumb I use in bike fit is no more than 2cm wider than your shoulders (measured from A/C joint to A/C joint). That means most women would be on a 36cm or 38cm bar. You should be able to lift your hands straight in front of your shoulders and grab the hoods while keeping your wrists in a straight plane (ie no cocked wrists). A bar that's too wide will cause pain in the shoulders/neck and potentially in the hands/arms (if you ride with a tight grip on the bar).

For performance, we tend to fit a bit wider, since this allows the rider to "push" the bar for high-speed cornering and descents. Because most racers spend much more time on their bikes than rec. riders, they're able to build additional strength that prevents the shoulder/neck pain. The other alternative is a bar that flares slightly at the drops, but this isn't necessarily safe for close riding (such as in a crit).