I had a pole barn shop about the same size as yours, well insulated. I used a wall mounted propane heater and it worked well. In the winter, I left it on full time, turned the thermostat down as low as it would go at night, and turned it up only when I worked out there. It was so comfortable I could work in shirtsleeves on the coldest day in the winter. Can't remember the exact size of the propane tank, but probably at least 100 gals. It would heat the shop for the season.