I have a 75,000 BTU gas ceiling heater, live in Wisconsin...garage is 27 X 35 with an 11-foot ceiling...all sheetrocked, with 2 36" ceiling fans blowing the warmed air down from ceiling. Garage doors 1-18x8 and 1-10X8, one window and one pedestrian door....I work in garage all the time...raise the temp from 35 (coldest when outside temp is -20f) to 60-65. I let it cool down when not in use....it use's fuel, but ALOT cheaper than electric !!!! My space figures at about 50,000 btu. Insulation is 12" ceiling which I am going to raise to 18" and 3.5 on 2.5 of the 4 walls (6" on remainder as it adjoins the house).
Stay away from electric as it eats your pocket book up....