10 W per square foot is what I used for an average house and that might also keep the temperature above freezing if the garage is not insulated but it needs to be airtight also. Insulation and eliminating air infiltration are the best things you can do to keep heating and cooling costs down.
My garages are well insulated but I had a wood finishing job to do a couple of years ago, in the winter, so I used shipping blankets and hung a curtain around my work area and used an electric heater inside that area and it did a nice job. Try to reduce the area that you need to heat. I’d even consider building some insulated portable partitions if you’re going to do that every winter. Craig