Garage Heater

Insulation ,and having any leaks for air stopped is most important.

I have a 3 1/2 car garage which is close to the size you mention, and all I have is 1 baseboard heater. When it's -25 C outside I can get to +4 degrees C

 

My garage is 26 X 30 and I have a little radiator with a fan to blow air (water system). I like air circulation. I also have a ceiling fan to force hot air down and it also help air circulation.

 

I have a Hot Dawg - they are really great!

Modine HVAC - Hot Dawg

 

I have one as well in my woodworking shop. A little noisy, but works very well.

 

How is a 21'x21' garage considered a 3 1/2?

 

Very important. I have a tiny Dayton in one corner, and you can feel the heat reflect off the insulated doors instead of going out. Then it shuts off, and the garage stays warm for a long time. Pull a car in, and the engine heat stays in the garage. Seal the garage up, then invest in a heater.

I have this one, and it more than heats a 1000 ft garage. With a good insulated garage, you can go even smaller.

DAYTON Electric Utility Heater,5/4.1 kW - Electric Utility Heaters - 3UG73|3UG73 - Grainger Industrial Supply

 

Not sure what the real standard is, but my old place had a two car garage, and it was a real two car garage ( forget the size ) my new 3 car garage at my new house has all 3 spots well oversized where putting the 3 of them together makes easily another half size at this house, if really squeezed the cars I could fit an old fiat 500 in here ..... I just referred it that was to the OP as I have to heat all of that
So, it's a 3 and a half

 

+1 on the Dayton, mine is a little bigger but not much... it runs 220v so its wired directly to my panel and I have a separate thermostat on it... keep the garage at 50 in the winter, and if i want to work out there crank it to 70 and in less than 30 mins its nice and comfortable..3-1/2 car garage