I agree the smaller ones won’t get it as warm... but they also don’t take up as much space, make as much noise, or make as many fumes...Don't know how un-insulated your garage is, but my 2 car is un-insulated with an open attic. A 23K round type kerosene would not suffice at all. I have a 140K BTU torpedo style and it heats it up pretty decent. It's loud I will say that, don't plan on listening to music with it running. There is a slight odor when it fires up and shuts down, but not horrible. I've only ran K1 undyed in it so far. It might be a little overkill though, they make 80K versions.
dont see your location but COAL is a great economical heat, +wood is even cheeper but more work unless you buy it. you could run a cheep hand fired unit or a small stoker stove with a thermostat that stays on. i live in pa=coal reigon + turn off my oil when its gets + stays under the 30's, about 2 ton for a heating season, a little less than $200 + haul it in myself + dump the ashes @ our local road cleaning ash pile. i am using a hand fired harman + nut coal but many other models can auto feed so it needs less care, but i am retired + mostly home or garage in the winter, garage 32-28 + well insulated uses a small wood stove THAT puts me in a TEE shirt in temps below 30 in short order!! check out coalpail.com + other sites for cheep heating!!