How large are the zones?
On the streets a zone is a variable-sized area edged by building walls and pedestrian crossings.
From the Zombicide rulebook (page 5):

ZONE
Inside a building, a Zone is a room.
On the street, a Zone is the area between two pedestrian
crossings and the walls of buildings along
the street. A single Zone may extend over two tiles
or even four tiles.

For example, in the following image, the red boxes identify the different zones of the map: