According to Krigger and Dorsi, the rule of thumb is 50 cubic feet of air for every 1000 btu per hour of appliance input.
However, before you start is you need to define the thermal boundary of the house. In other words, what part is insulated and conditioned? For example, think about the attic insulation. The part above the insulation is outside the thermal boundary. If the garage is outside the thermal boundary, then it probably won't do much good to do crack sealing and such in there.
One way you can test to see if the walls are insulted behind the dryway is take a metal insulation support, or any strong, thin piece of wire, and work it through the drywall. When you pull it out, it should pull out a little fiberglass with it.