I've have installed vented garage heaters in two homes. Both garages were insulated, but not as large as your shop. I used the 30,000 BTU size and it was more than adequate. They are also available in 45,000 and 70,000 BTU sizes, I believe. 45,000 BTU would be enough if you gave it some time to get the place warmed up.
My only complaint is they are relatively noisy in a quiet shop - a squirrel cage blower would be far less noisy compared to the propeller fan.