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There are many times when a green screen is not large enough to cover the entire subject. You end up using a combination of green screen and rotoscope. In your car driving up to the boat example you would position the blue screen so that the blue screen was behind the windows of the car because that wold be the hardest parts of the car to rotoscope by hand. It would be fairly easy to rotoscope the wheels and the bottom of the car.
Take a look at this video from Lynda.com - a really great source of After Effects Training. It will show you how multiple masks are used to clean up a green screen when it does not cover the entire scene.
There was a great commercial for Singapore Airlines called Across the World that had an actress walking continuously through a shot that seamlessly moved her from San Francisco to Paris to Venice and in all of the transitions she walked past a green screen that was just big enough to cover here head and shoulders because it's very difficult to rotoscope hair and hands when they are moving. I took a quick look for the behind the scenes video that was produced when they originally released the commercial but couldn't find the exact one. If you look closely in this video you'll see folks moving green screens around as she is walking.
This technique is even used on blockbuster movies. Note the green screen here that doesn't hide the entire background, just the parts that would be very difficult if not impossible to rotoscope.
Thank you so much for the help! I will definitely look more into "rotoscoping"