Depending on your location, when first starting the Waze application, a box appears at the bottom of the screen for a few seconds. It displays the number of Wazers nearby and the number of road reports in that same area.
The area is a polygon covering your current metro area which is usually the main city for that location and the satellite cities (suburbs) around that main city. This data is not part of the current city layer (or any other layer) that editors can effect.
Each country has about 15-30 of the largest metro areas defined. In the USA, the USGS metro shapefile is used to generate the polygons. When that data is not present it is drawn manually or the USGS urban area shapefile is used.
Metro areas generally have a radius as large as 31 miles (50km) or as small as 6 miles (10km). In more densely populated areas this can be two to three times larger with differing shapes.