I always rent Full-Size vehicles. When I rent, it normally follows this routine if they are out of Full-Size.

Offer a Prius.
If they don't have a Prius; Premium.
If they don't have a Prius or Premium; Mid-Size.
If they don't have a Prius, Premium, or Mid-Size; Luxury.

The last two times I've rented, Avis was out of Full-Size, Prius, Premium, Mid-Size, and Luxury. When that happens, they offer you a cool car, which I was glad to take!