To determine the size of the air conditioner that you need first measure your window so that you can be sure that the air conditioner is the right size and it will be the perfect fit for your window. When buying a window air conditioner you also want to consider the BTU (British Thermal Unit) which determines the air conditioners ability to cool or heat specific rooms. Window air conditioners with 7,000-8,000 BTUs are recommended for rooms up to 250 sq. ft.