The size of the window conditioner you choose should be just right for your room. Please find out the room area in square feet and compare it to the AC’s BTU (British thermal unit) rating. The BUT generally indicates the amount of heat an AC can remove from a room based on the heat source (stove). So if you have a large room, the AC should have a higher BTU rating. A 150 to 350 square feet room matches well with 5000 to 8,000 BTU, while a 1,050 to 1,600 square feet room goes with 18,500 to 25,000 BTU.