I think I would have gone with a larger BTU heater. I have a garage that is 864 sq ft (24X36) and I have a 60K BTU Hot Dawg heater. If it is really cold and windy and I open the overhead door for whatever reason, it is nice that I can warm the space back up rather quickly.
I try not to open the overhead door if at all possible, but if I am bringing in some plywood or some other large pieces of wood for a project, or if I am bringing in a large project to work on, I may need to open the overhead door. When I do open it a lot of the heat goes outdoors. At those times having a large enough heater to bring the temperature back up quickly is a big plus. It isn't totally necessary of course, but it is certainly a big plus.