I have a 520 sf attached garage. I used the heater below and it works great. Needs a 30a breaker, 220v. Ceiling mounted. Heats up the room almost instantly. OK, maybe 5 minutes.
Fahrenheat 5,000-Watt Unit Heater-FUH54 - The Home Depot
Provide 5,000 watts of heating power to up to 500 sq. ft. of a garage, basement and warehouse or construction addition using this Fahrenheat 5,000-Watt Portable Electric Heater. A high-level cut-out automatically
www.homedepot.com