Whatever heater you choose, put in ceiling fans to push the warm sit back down to circulate and even out the temp. Otherwise it will just rise and heat the ceiling.
X2 on the above. My shop is in SW Washington, winter temps are not severe but rarely warm. Highs in the 40's-50's, lows in the 30's most of the time but it can get a bunch colder at times. It is 30x40 with a 10' wide built-in room down the long wall making the actual floor space in the shop 20x40 with 13' walls with an 18' peak. I had it fully insulated, which is the best money I've spent so far to make it comfortable. I added a pair of 60" fans from Home Depot to push the warm air down. All it takes to keep the place shirt-sleeve warm in any normal weather is a single 30000-60000 BTU propane heater. If it gets really cold I will add another propane heater. My shop has an open ridge cap and leaks air like a sieve at the sliding door. Still, warm and cozy with the above mods.
