The size of the AC unit will probably depend more on how well the garage is insulated than the size of the building. A well insulated building will keep itself cool during the hot part of the day. An unsulated one will heat like an oven during that part of the day.
There is a rough guesstimate that is about 400 sq feet per HALF TON of cooling. You don't to go too large, this will cool the space but not dehumidify the air. A qualified HVAC company should give you an estimate.
