It is important to know how big the room in which you are going to be using the device is. If the doors to other rooms are open, the area of those rooms must also be included. Room area is calculated by multiplying the length and width of the room (e.g. 5 x 6 metres = 30 m2). To get the room volume, you multiply the room area by the ceiling height (30 m2 x 2.5 metres = 75 m3).  The room size should be doubled for minimum energy houses with controlled ventilation.  Choose a device that is recommended for at least your desired room size/area.