We were at a crowded restaurant the other night and got stuck in an outdoor overflow area. It was enclosed but barely. They had 3 of those big mushroom radiant space heaters going. It was cold enough you could see your breath, yet was able to sit around in shirtsleeves if you were close enough to the heaters. Radiant heat would be the way to go.

Dad's wood shop is about 26 x 40 and he heated it with a wood burning furnace. It had a blower, but no duct work. Just forced hot air into the shed. Worked, kinda sorta, but burned a lot of wood for the amount of heat it produced. If I was to replace that, it would be with some form of rocket mass heater......stacked fire brick combustion chamber, then run thru the 55 gallon steel drum, then thru a thermal mass like a brick bell. Or a commercial built unit like the Liberator Rocket stove, with exhaust run thru the thermal mass. Both of those are going to radiate heat for hours, and if it cools down, fire it again.

This isn't what I'd build, but gives you the idea of what is going on. Close to 99% combustion of all smoke and gases.......fire gets to 1,500*F plus, but by the time it gets to chimney, is down to 150*F to 200*F. The heat is stored in thermal mass to radiate out over time.

We use a 250,000 BTU Kerosene heater torpedo style for our 30 x 60 Metal facility. Works great to heat it up pretty fast. Then I switch to a 55,000 with less noise after we are heated up nicely. We also have a wall heater propane type 30,000 BTU for help as needed to maintain the temperature after the torpedos are shut down.

You will most likely have a major condensation problem if you let the heat hit the inside of an un insulated metal roof. The building we have on the go in Canada is 22x64 in the living space. Looked into spray and blow in insulation. Couple builders recommended 1” ridged foam on bottom of trusses for ceiling and as many inches of blow in insulation as you can afford blown on top of foam panels. Ridged foam acts as vapor barrier and isnt much more $ than wood. Blow in is less than 1/2 the cost of spray foam. Be sure to use chutes to avoid plugging soffit vents with blow in.

We plan on heating main area with wood stove and comfort cove wall mount radiant heaters in bed rooms and bathrooms.

put in a wood burning stove,

One thing about spray on foam is when the steel rust from the inside you have to replace it and the insulation, I know the shop is a mess because of this.