Which rental company did you get, and what have you been told is their selection of full size vehicles?
Full size vehicles can vary significantly by company, but the smallest vehicle I'm aware of considered full size by any of the five you could have gotten is the Chevy Malibu (Avis), but Avis has other vehicles available in the same class that are larger (Lumina, Impala, and in Orlando the Pontiac Aztec).
Hertz - Taurus (Accord and Camry in some cities)
Alamo - Buick Century, Lumina
Avis - Lumina, Impala, Malibu, Aztec (in Orlando)
National - Lumina
Budget - Tauru