In my 24x26 two story garage (11 foot first floor ceiling), I run a forced hot air mobile home furnace on a cottage base (vented on three sides). I keep the heat at 50 year round and turn it up to 60 when I am working in it. Runs off of Kerosene in a 275 gal tank outside. I fill the tank once every two years.
The cottage base is nice since it blows out on the floor and rises from there, heating the concrete floor in the process.