David-My pole barn is 30x44 with 14' side walls. Originally there was no interior walls, ceiling, or insulation. I heated with a 110,000 torpedo heater but all the heat went to the rafters. I now have interior side walls, ceiling, and insulation in the ceiling and walls. The torpedo heater now heats the building nicely. A retired friend just finished hanging a 40' Rayvac radiant propane tube heater. Will fire up as soon as the propane tank arrives in a week or so.. We have 4 radiant heaters at our truck service shop and it works much better than the old forced air we had. I would want at least a 30' heater in a 40' building. You can buy a 40'er and cut off a couple foot. The burner end puts out a lot of heat but gets less as the heat goes down the tube.