Viggen:
Here is the link to the museum in Cologne. When I look at that coach (which will be pictured on the museum's website) they look wider than the road tracks and the coach's cabin. And yes, if you visit your local museum in June, if you come across any information about vehicle proportions, please inquire. As you may already know, there were a variety of different sized vehicles like we see in our cars today. When I was walking toward my SUV the other morning, it struck me that from a distance I was reminded of the image of the Roman coach. Haha. :-)
http://www.museenkoeln.de/roemisch-germanisches-museum/pages/4.aspx?s=4