There are many sizes of air conditioner available, not just physically but in terms of power as well. An air conditioning unit that has an output of 8000 BTU will effectively cool a room up to 350 sq ft. If the room is smaller, energy is wasted, and if the room is larger, the unit will not produce the desired effect.
Because of the size of room this type will cool, the units available are usually window mounted. Both Whirlpool and LG have models of this size that are very well respected in the marketplace, and although they are large brand names, their units are very affordable.