Maybe some that are more educated on this subject will chime in
My shop is 30x40, block construction of which about 1/2 is buried into a hill. the center stall ceiling is vaulted to allow use of a lift while to other two bays are at 10 ft. Very well insulated.
So far I've been able to get by with 2-4 oil filled radiant heaters. I start with 2 in the fall on low when the temps begin dropping in the bldg. As the outside temps drop I ramp them up, add a 3rd and the 4th when it starts getting down to 0 or below.
Summertime is handled by a 25K? BTU window A/C unit.
The existing setup works pretty well but am always open to improving things especially if there is a substantial cost reduction.
And just an FYI, I am all electric with the house and shop.