Also, why would we want to know this? NASA’s Vital Signs of the Planet feature How wide are the world’s rivers? by Adam Voiland (7/18/18) answers the questions.
Most scientists who study rivers rely on measures of discharge, the volume of water transported through a given cross-section of a river. Much less studied, though critically important, is a river’s total surface area, particularly for scientists trying to understand how carbon dioxide moves between rivers and the atmosphere.
The work has resulted in a global database of river widths. The map here, copied from the post, answers the main question: