Hey Gary, here's a couple photos of the old town of Marburg, about an hour's east of Frankfurt. It's an old historic town, so the streets are pretty much original width. The first one shows the sidewalks and street-typical for an old European town, from my extensive world traveling...
However, what Manny has said is true as well; there is no "officail" standard width, nor for sidewalks either. Some streets have them, some have mere footpaths, others--like the Roman-era original road here going up to the Marburg Schloss/University (the old castle) have none to speak of. You can see the relative width in this shot...
I would just go for about a scale foot on either side of a Pz IV for a small town, and increase that to about two scale feet on either side ( + or - for the setting) for a larger town. I hope this helps you out!