There are different kinds of motion sensors. The ones most commonly used as standalone sensors with inexpensive home automation systems are “PIR”, “passive infrared.” They are detecting heat, but it is really minute changes in heat. For example, a cloud going across the sun can create enough of a temperature differential that the sensor will trigger once the cloud has passed. This is also why this particular type of sensor is prone to false positives.
As far as the size of the detection zone, the first rule of home automation applies: “the model number matters.” Different models have different size zones, anywhere from about 3 m to about 20.
The ones that are built into light switches typically have a detection range of about 5 m, intended to catch someone walking into the room.
see the FAQ:
FAQ: Where to locate motion sensor for fastest response?
The motion sensors which are used with cameras are typically a different technology using video analysis. those are true motion sensors. 