I have some good and bad news about the green movement. The bad news first... If you buy a car that is very small and hit a car that is bigger and/or heavier, the smaller vehicle will likely be worse off than the larger/heavier vehicle as a result of the impact. So if you are being exceptionally green by picking a car based on small size, you are more likely to get hurt or killed in an accident.
Granted, if you have a typical midsize car and hit an SUV the same principles apply.
Here is a great article and video on the matter. A must watch if you are considering a purchase of the smallest car you can find. Quick details: See a Smart Fortwo hit a Mercedes C class and in doing so fly into the air while the engine compartment disintegrates.
Let's just keep in mind the pitfalls of decisions we may make in life. One of the best lessons I learned at the University of Connecticut was from my chemistry teacher's assistants when they told me, "Everything in moderation... Including moderation."