I have a 100,000 btu modine ceiling propane heater in my well insulated 24x24' shop. More than enough heat. I like the forced air type heating and can be in t-shirt in 10 minutes after I start it. For your size shop and with good insulation, 150,000 would probably be ok but I tend to oversize them for rapid heating. I bought a low temp thermostat and keep it set above freezing in the winter. You can get them with pilot light or electric ignition so it uses less fuel. I use a pilot light model since the heat from the pilot light also helps heat the shop when turned down low. They also come with optional power venting, which allows direct venting out the side of a building instead of up through the roof.
Lots of people prefer the propane radiant heaters but for me these take longer to heat the shop. However, the heat produced will heat the surrounding equipment, wall and floors. This makes for a more pleasant type of heat.