It depends where you have placed the monitor.
The main limitation to the ability of a detector (such as our Corentium Home) to measure radon is not really how big the room is, but it is rather the presence of physical dividers, like doors and walls.
In theory, even 2,000 ft²/ 200m² (if not more) can be fully monitored by one single Corentium Home: in an open space the air mixes continuously, so the detector always measures a relevant sample of air.
The air sampling becomes more difficult when there is a door, or a wall, or any other physical barrier that divides the room in different areas - like cubicles in an office. In this case one radon monitor may not be enough, so you either perform multiple measurements with the same detector or you may want to purchase two monitors to check the air quality in two different areas at the same time.