Motion sensor large areas
A standard PIR motion sensor will work fine.  It can monitor motion in a very room.  You just need to make sure the PIR sensor is far enough away.

Here is one article that states it should work up to 10 meters away.

https://www.mobotix.com/eng_US/Support/FAQ/Configuration2/What-is-the-range-of-the-PIR-Passive-Infrared-motion-detector

Note that the device that the PIR sensor is mounted on must be stationary.  It will not work on a robot of anything that is moving.