One alternative for a shop your size is a 'pot burner', especially since you say you don't use the stove when you're not in the shop. I've had a couple, and I'm selling one now (Thermobile AT307) that would probably run you out of the garage. They're more work, you have to light them manually using diesel to get going, and you have to clean out the pots (I've found using industrial aluminum foil to line the bottom of the pots makes it easy) but they put out a lot of heat and burn pretty clean once they're going. I just found I'm using my shop more lately and I'm upgrading to a regular thermostat operated furnace to save warm up time. I don't know what the smallest unit you can get is on a pot burner, they're getting tough to find.
