Originally Posted by CrazyOne

When you pick up the car, just go ahead and try to fit the luggage. If it doesn't fit, then you can talk to the agent to maneuver into an upgrade of some kind. As long as you are picking up and dropping off at the same place, an on the spot upgrade may work out. If you're taking the car one-way and dropping it at the other end of the country, they may be less inclined to give you anything different, but still you never know.