Try playing around with some cardboard and see if you can find a good length for coverage and still be able to easily take the wheel/tire off. This is where I ended up on my Neon, I went about as far as I could go, and still reach all my lug nuts, and put the wheel on or off without needing to take of the skirt. I also had an aluminum strip on the bottom for structure and support, so it limited me from going too much lower as well, without having to bulge out away from the car.
