My shop is only 112 sq ft. I have one outside wall, one common with the kitchen and two walls common with the garage interior. In even zero weather, which is rare, my shop never gets below 45 degrees. However, I like it warm. I purchased a 220V 30 amp heater which is supposed to heat a single car garage efficiently. That is what they say. The heater is a Dimplex brand and is a piece of crap. I installed it this past spring and have now had a chance to use it. Very inefficient. I does not have high and low settings just a rheostat. When turning it all the way up it soon goes into default. If turned up to 3/4 the heater does not run long enough to heat the shop. I can fix the problem by disassembling it and moving the internal thermostat a little further from heating coils. That will work but also be potentially dangerous if it does not shut down if over heating. I'm back to my parabolic heater which works but I don't like having to always walk around the thing. DO NOT BUY a Dimplex heater!
OK, I'm done now. :>)