Reserve the size car you need. Some guidebooks suggest reserving an economy car. They say rental agencies rarely have anything smaller then a full size car.
This advice is no longer valid, and was never valid for Orlando. Many of us have observed famlies trying to fit a lot more luggage in the trunk of an economy car then could possibly fit. Reserve a larger car and the extra cost may be nominal. Wait until you need it and you're at the mercy of the rental agency. Worse they may not even have a large enough car available.
PP has a point, sometimes the rental agency will offer a great price on an upgrade. Say no and you may get the upgrade for free. Othertimes it's just a matter of inventory. They know they won't have enough smaller cars for all customers. They'll start by trying to get customers who're willing to pay for the upgrade. Say no and you may wind up with the car class you reserved.
Back to the main point reserve the car class you need. Smaller cars have small trunks.