Room size is dependent on the square meterage of the space. You can determine the size of your unit by multiplying the length x width of the room, then multiply this figure by roughly 600 Btu’s per m2. Each size unit on the website specifies the square meterage of the space it caters for.
E.g. A room size of 20m2 (5m across by 4 meters wide) will require a 12000btu size aircon. A room size of 40m2 (10m across by 4m wide) will require a 24000btu size aircon and so on.