Small Kit: Designed for ATV's, 4-Wheelers, Side-by-Sides or similar. Not suitable for cars or trucks.
Medium Kit: Designed for Coupes and small Sedans. (Porsche 911, Ford Mustang, BMW 3 Series, or similar).
Large Kit: Designed for larger Sedans and small SUV's. (Audi A7, BMW 5 Series, Mercedes S Class, Mazda CX-5, or similar).
Extra Large Kit: Designed for most Trucks and SUV's. (Ford F-150, Audi Q7, Ram 1500, Ford Expedition).
If in doubt, please upgrade to the next size up. It is always better to have a little extra material vs having not enough. Super size trucks or SUV's may require addition material over the Extra Large.