Right now I have a unit that came out of a hotel room. Its only 4800 watts. It does ok, at taking the chill off, about 58 degrees. My shop is 620 sq ft fully insulated with 8ft walls except ceiling (on my list).
The new one is 7500 watts, so I'm thinking it should be able to heat up to 65 and keep it there so I can epoxy prime.