A heater's physical size has little to do with its heating abilities. The heater type, such as radiant or forced air, the British Thermal Unit (BTU) output, your garage's insulation and your available power all factor into your heater selection. For fast heating, choose a forced air over a radiant heater, as forced air heaters will make you feel warmer much more quickly, while radiant heat sources may take hours to heat a room to a comfortable temperature, although they provide a more even heat in the room.
Measure the length and width of your garage. Multiply them to get the total square footage of the room. An average two-car garage is approximately 484 square feet (22 by 22 feet), but it may range from 400 to nearly 600 square feet.
Check your garage's insulation. If your walls are insulated the same way as your home and you have a finished ceiling that is also insulated, you can use a smaller (fewer BTU) heater than if you have no insulation or open rafters, which accelerate heat loss.
Divide your total square footage by 200, and then multiply your answer by 9,000 to determine the BTU requirements for an uninsulated garage. Multiply by 6,000 instead of 9,000 if your garage is insulated.
Find a heater with a BTU output that matches your results. For example, if your uninsulated garage is 484 square feet, your formula is 484 / 200 x 9,000, or 21,780. You need a heater with at least a 21,780 BTU output.