Why not tell us what you have and we can give you a best guess?
Size of the garage, type of construction, what part of the country you live in?
So about an R10? 18,000 BTU should it, 24,000 BTU will for sure.Metal tubular building.
930 sf
Currently insulated with radiant barrier and 2” foam board.
Central FL
So about an R10? 18,000 BTU should it, 24,000 BTU will for sure.
I have a stick framed garage about the same sqft, 2x4 walls and crappy attic insulation with 18,000 BTU in North Dakota and it does just fine. I had a 12,000 BTU window unit that would keep up as long as it was started early in the day.