The typical dimensions of a garage depend highly upon the intended use. Making the assumption that the garage will be for two cars and include space for some storage, the typical dimensions would be between 20 and 24 feet wide and between 20 and 24 feet deep.
To determine the best overall dimensions for your intended use, there are several things to keep in mind. Sticking with the example of a two-car garage, are the cars compact cars or full size trucks? A good estimate for depth is 14 feet for an average car, 18 feet for a small truck or SUV and 20 feet for a full-size truck plus space for storage. A typical car is 6 feet wide. Providing enough room to open the doors on either side of the car requires the garage to be a minimum of 12 feet wide. For a small or full-size truck, the suggested width is 14 feet. If a two-car garage is planned, simply double the widths above for a good suggested width.
Another point to consider is the width and height of the door (or doors) on the garage. For a single door on a two-car garage, the options are usually 16 feet or 18 feet. If you are using two doors, 8 feet wide by 6 feet 8 inches high are sufficient for the average car. However, for trucks, or simply for extra room, a 9-feet wide by 7-feet high door is recommended.
When planning your garage, there are many other things to consider such as the amount of storage space desired, ceiling height and attic access.