my workshop is roughly 18 x18, with no ceiling and no insulation, so you cna say that to the center of my roof it is 12'. i use a Kerosun 85 and it works good. it DOES take a while to heat it up, but once it gets there it can easily keep the shop comfy. it will not heat it to 70, but it will make the mid 60's no problem. a lot better than 30 without it!

2 words wood stove. I had a similar setup in my old house. kerosene heater made it tolerable in the winter but you were still cold. Looked around and found a used small wood stove. Got all the piping for around 150 bucks from Lowes or HD. Wood was free just had to go cut it. It could be 20 outside. I would be in the garage working in a t shirt and be sweating. It was also not insulated.

you have to be aware that kerosene heaters use a good bit of O2 in the room. As well as exhausted CO2 into the air. Wood stove is a much safer bet if you work in there for any length of time.