If you have a 700 square foot garage, you'll need a heater that can produce at least 35,000 BTUs to heat the space. There are a few different types of heaters that could work for this size garage, but propane and electric forced air heaters are the most common. Make sure to choose a model that's specifically designed for garages or other large spaces, as regular home heaters might not be able to adequately heat the area.