To the OP: Unless a premium rental is not much more in price, I would reserve a full size car -- that doesn't necessarily mean you'll *get* a full size car, but you are extremely unlikely to end up in something smaller. Most of the full size rentals should hold your luggage, but some will do so easier than others.
As others have suggested, when you get to the airport see what they assign you -- and you can ask the agent for a car with a large trunk when you are signing the paperwork. Then go see the car and if you think it will meet your needs. Likely you can upgrade, but they'll try to get as much out of you as possible for the upgrade (and even at $10/day that will add up significantly for a long rental) but generally speaking a full size will work for you.
Good luck!