I borrowed a 13'000 btu oil fired/hot air furnace from a building that was being torn down
.That was 12yrs ago
.But anyway this is what I use.It's nice because when it's cold(northeast) I set the thermostat for 50 degree's'
.Garage stays nice and warm.I put iol in the tank when I fill my house.Nicest thing is when I want to work on a project I just turn up the thermostat to 60degree's and with in 10min I in a tee shirt.My garage is 24'x30' with 10' ceilings.6" walls insulated and sheeted.So it also depends how good you insulation is.Thats what I have to offer. Stay warm,