According to the Energy Star program for energy efficiency, a 1,100 square foot room requires around 21,000 BTUs per hour to cool. The number of BTUs (or British Thermal Units) required for an air conditioner unit to cool a room depends on many factors beyond the room's size.
Some of the factors affecting BTU count include the amount of sunlight, the insulation of the room and the use of the room. A very sunny room will require about 10% more BTUs on average, while a shady room will require 10% less. The air conditioner used should be as close to the recommended BTU number as possible, since one that is designed for a larger area will waste energy and potentially leave excess humidity in the room.