A good manufacturer of bars, such as 3TTT(.com) produce them in a variety of widths, drop radii and curve styles. The clamp diameter is also given in their geometry charts, and they make stems to match, in a variety of length.
The usual range of bar width is from 36 to 48cm, with the "std" being 42cm. There is no "correct" way to size a bar, but many people match them to the width of their shoulders, measured at the center of socket joint.