If you've got natural gas your set.
I had something like this (with the blower) in my old garage. It was smaller than yours is now but could run you out of there.
FREE SHIPPING Mr. Heater Vent-Free Natural Gas Radiant Wall Heater 30,000 BTU, 5-Plaque, Model# MHVFRD30NGT | Natural Gas Wall Heaters| Northern Tool + Equipment
With the size of your garage I might try one of those box shaped heaters that hang on the ceiling in the corner. Lots of options on Craigslist and most of the stores will be or have already put that stuff on sale this time of year.
I've got the "blue flame" propane version of that wall heater now. I'm ready to ditch it and get another one of the brick/infrared style. They work.
ps. Rafael
@Roxx is the man. He spent a long time on the phone giving me some great advice on getting our new heat pump a couple weeks ago.