My bike came with 3T ergonova pro 40cm bars and they were definitely too narrow. I recently changed the bars to 3T ergonova team 44cm. I just changed them this past Saturday and haven't gone on any long rides though but in just the short test ride around the block they "seemed" more comfortable at least. I usually start getting some numbing on my wrists after about 20 miles so I'll see on my next longer ride.

As for fit, if you have the luxury of taking the bar off (or shopping at a store with lots of bars), if you hold up the bar against your back shoulders, you can sort of get a decent idea on width. General rule of thumb is to get the bars so that your shoulders fit inside the bars when held up but not too tight.

Sort of like this: http://miamirally.com/cm/images/stories/08BikeFit.jpg