An automobile is about 16 feet long - depending on make and model. You can take a tape measure and find out how long your car is - bumper to bumper. You need to have that length in the driveway so you can stop the car and close the door. If you want to know how much room is required so you can exit the garage and then turn - you must be able to have the driver outside the garage so they can see on coming traffic plus the width of the street or alley you will be turning onto. This would be a minimum of 15 feet plus the width of the street. If you know an architect or designer you can look at their Architectural Graphics Standards book which will provide the diagrams for all the possible options. Unfortunately it is a copyrighted book so I cannot post a copy of the page on-line. An alternate is to take your car to a shopping center with a piece of sidewalk chalk, mark the "garage" around your car and then "back out" and mark where the car is stopped. Take a measuring tape and you will have your dimensions.