Although it is difficult to suggest the ideal BTUs estimate, however, most experts suggest that a minimum of 45,000 BTUs are ideally required to keep the entire two-car garage warm. If you are searching for a warmer or maybe more comfy temperature, 60,000 BTUs is also an ideal BTU estimate.
You also have considered that, although 45,000 to 60,000 is an ideal BTUs estimate to heat a two-car garage, the quantity of BTUs also depends on the heater type you wish to use.