The Fahrenheat unit has been covered on this forum quite a bit. If you use the search feature on the Heating and A/C page and search for the words "Fahrenheat", "Marley", or "Dayton G73" (same heater, different brand names) you'll find many threads.
My attached garage (ceiling and garage door are insulated) is about 24'x26' and the FUH54 does a good job. I only turn it on when I plan to be in the garage for extended periods, and it takes it awhile to bring the heat up to a comfortable level (half hour or so depending on how cold it is).