One thing you have to watch out for with Car rental agencies is that their idea of a full size car is what most people consider to be mid size.
A Mid size is usually a Pontiac Grand Am, Chevy Malibu, Mazda 6
A full size car is usually a Ford Taurus, Pontiac Grand Prix
A Premium car is usually a Ford Crown Victoria, Pontiac Bonneville, Buick LaSabre.
Often times, the trunks in cars vary greatly. On car rental websites, there is usually a diagram that tells you how many people and luggage fit in the car.