road bike on the right of the fold width is the width of the shoulders.
Just with her hands high on the side grip arms must be perfectly aligned and parallel to the shoulders. If you do not have access to a handlebars of the right width is preferable to adopt a slightly larger. With a handlebars larger you get a better breathing, but it worsens the aerodynamic efficiency. In contrast, a narrow handlebars the shoulders prevents proper breathing is too heavy-handed. Another important element is the height and depth. These two elements must be identified so as to allow the body a proper riding position, which obviously will be verified with the biomechanical. If, however, you want to choose these two parameters independently, consider that a high height and depth, leading to assume a very aerodynamic, but also more fatiguing. While a compact handlebars, thanks to reduced height and depth, allows a more comfortable ride at the expense of aerodynamics.