Of the three points of contact with your bike (handebars, seat, pedals), the handlebars offer the widest selection of styles. As Dan has said, your choice of handlebar style is highly personal.
I prefer straight bars over drops, and when I built my Nomad last year, I tried several different bars until I settled on the Thorn Flat Track bars. They have a 10 degree sweep-back which I find perfect for my wrists. But relevant to your question, they are a 640mm wide and have 10mm markings on each end, which make it easy to cut the bar down to your preferred width. Since the bars are straight, from the center bend to the end of the bar, it is easy to adjust your grips/brakes/shifters in or out until you find the perfect width that fits you. I currently have about 5cm of bar sticking out past the grips on each end. I should get around to cutting it down sometime, but I like the flexibility of moving them later if I want. Of course, if I want to have bar-end grips, I would need to settle on a width and cut the bars.
If you prefer straight bars (as opposed to drops), the Thorn Flat Track bars allow you to experiment with the width until you get it to your liking.
One more point: Hold your arms out in front of you as if you were riding. Move your hands together or apart. Notice that your hands move in an arc, not straight in and out. To me, this suggests that if you have wider handlebars, you probably want more of a sweep-back angle than if you had narrower bars.
- Dave