Building code ventilation depends on the building use. For a shop where you're going to work on carbs, etc, where gasoline fumes might occur, higher ventilation rates are required. For condensation, it's a matter of keeping the inside surface temperature above dew point temperature when it's colder outside than inside.
I know that doesn't make much sense, but I can check the code books later in the day and give you a recommendation.
Going from memory, typically, 1/2 air change per hour would take care of it. If building volume is 13440 CF (that assumes a 12' ceiling height for your footprint), a fan capable of 120 CFM (a bit bigger than a bathroom fart fan) would get that air change rate. Turbine roof vents like Martin suggested would do the trick as well. You also have to have somewhere to let air in (try sucking air out of a bottle).
