Air Conditioner Sizing Tutorial. How much air conditioner BTU's I need? Picking the right home air conditioner is very easy with the right information. Before buying an air conditioner, find out how many BTUs you need. This means think of the size of the room, and what the room will be used for. Matching Btu requirements to room size is very important when you buying an air conditioner system. This air conditioner sizing tutorial will explain you how to choose the right air conditioner size.