EPA size categories, based on calculated passenger + cargo volumes, are a lot less charitable than rental agencies:
Toyota Corolla, Mazda 3, Chevy Cobalt = compact
Ford Fusion, Chevy Malibu, Toyota Camry, Nissan Altima, Mazda 6, Buick LaCrosse = midsize
Chevy Impala, Ford Taurus, Hyundai Sonata, Chrysler 300, Dodge Charger, Honda Accord = Large
There are a few surprises: Honda Civic = subcompact, Nissan Sentra = midsize
But, unfortunately for the OP, Altimas, Camrys and Malibus are often rented as Full Size class vehicles.
http://www.fueleconomy.gov/feg/findacar.htm