📐How big is this room, and what does that mean?

Multiply length by width in feet. A 16 by 12 foot room is 192 square feet. If the room is not rectangular, divide it into rectangles, work out each and add them together — that is more accurate than any single measurement and takes another thirty seconds.

To the wall, at floor level, which in practice means to the face of the skirting. The difference is an inch or so per wall and does not change any sizing decision. What does change it is measuring inside furniture or leaving out an alcove, both of which can move the area by ten percent.

The average: add the height at the lowest point and the highest point and halve it. That gives the right volume, which is what matters for air changes. For heating and cooling, a room with a vaulted ceiling behaves slightly worse than the average suggests, because warm air collects at the peak where nobody is.

If air moves freely between them and there is no door that stays shut, yes — they are one thermal space, and sizing only the room guarantees the unit runs continuously. Stairwells count too, and count for more than their footprint: warm air will use them.

Divide by 10.764. So 192 sq ft is 17.8 m². For volume, divide cubic feet by 35.315 to get cubic metres. The tool shows both at once, because equipment specifications routinely mix the two — a European heat pump sheet in kW against a room measured in feet.

Because the published charts were built around the standard residential ceiling in the United States, where the rules originated. That is why every tool here scales its result by ceiling height divided by eight: the underlying load follows air volume, and the charts only ever measured one height of it.