This is the heater I have:
http://cgi.ebay.com/55-000-BTU-KEROSENE-REDDY-HEATER-used-works-fine_W0QQitemZ180299016719QQcmdZViewItem?hash=item180299016719&_trkparms=72%3A1205|39%3A1|66%3A2|65%3A12|240%3A1318&_trksid=p3286.c0.m14
My garage is about the same size as yours, and pretty well insulated. After about 15-20 minutes of it running (with it being subfreezing outside) I have to turn it off because it just gets too damn hot.
Since my garage is attached, and since I smoke in the garage instead of in the house, I am constantly flipping it on, so the garage rarely gets below 40 degrees. For your garage, you may want to step up to a 75,000 or 120,000 btu heater, but I REALLY recommend sticking with this style of heater. Construction workers use them outside, if that tells you anything.