I'm pretty sure that Enterprise follows the same fleet structure as National. The Pontiac Vibe is a twin to the Toyota Matrix, which I know is a STANDARD size car, one below Fullsize.

Some Fullsize examples include:

Chevrolet Impala
Chevrolet Malibu
Dodge Charger
Mitsubishi Galant
Nissan Altima
Toyota Camry

There are others but those are some of the more common ones. The Ford Fusion and Hyundai Sonata are both residents of the Standard class, but as two of my favorites I often find myself taking them on Fullsize bookings since they're similarly-sized and even better equipped than some of the Fullsize offerings.