The size of a fire alarm panel is determined by a number of factors, including the size of the building, the number of devices, and the job’s requirements. A zone is a defined area in a building in which related functional items work together. This could include things like alarm annunciation (the panel will indicate what area of the building the alarm occurred) and alarm notification (signals will only activate in particular areas of the building, not general alarm).
On a conventional fire alarm panel, the terms zone and circuit are often interchanged (one circuit could cover one zone). On an addressable panel, the zones are created in software (you can assign devices to zones). Circuits on a fire alarm panel also have limits as to how many devices you can connect to them. The more devices, the larger the panel. It’s also up to the engineer designing the system as to how he wants to divide up the building.
A single zone conventional panel, for example, may be able to support enough devices to cover a given building, but you would be limited in terms of annunciation at the panel because you won’t be able to tell where in the building the alarm came from.