One of the biggest complaints people had when air conditioners started to appear on the market was their (in)ability to effectively cool the surface area required. This is why we have so many sizes today. Among the smallest are those that have an output of 5,000 BTU. BTU stands for British Thermal Unit, and is measured per hour.
An air conditioning unit of 5,000 BTU will normally cool an area of around 150 sq ft.
Frigidaire offers one of the best window mounted units of this size. Keystone also has a highly revered similar model that’s Energy Star rated. Both manufactures are incredibly affordable and offer great warranties.