Buy bigger than the space needed and natural gas if an option. I have a large 3 stall with 10ft ceiling and the 100k natural gas heater has no trouble getting it up to workin temp from 50 (where I leave it).
I just put up a 125,000 btu Hot Dawg it works great and is quiet. A friend of mine put in a Mr Heater and it looks to be poorly built. It vibrates so bad it is shaking the rafters, appears the fan is out of balance. It is also noisy.
I have been using dr. heater DR966 because it comes up with a thermostat. So far it's working good for me. Highly recommended for small size garages. Even if you have a large size garage then you probably have to look up for some other options. You can visit Garageaxs to read their best garage heater review.