First, be aware that the Preferred Plus one-class upgrade benefit is not available on rentals exceeding 28 days. I would keep the reservation below that threshold and extend or split into two from the outset.
Then, check the the upgrade thread:
http://www.flyertalk.com/forum/avis-...-upgrades.html. There's a good deal of info in there.
As a general rule, you can expect to receive a vehicle that's grouped one or two classes higher than what you reserved.
I usually reserve a midsize SUV (Group F: Ford Escape, Toyota RAV4) and have driven off the lot in Kia Sorentos, Mazda CX-9s and Ford Explorers - that's one and two classes up from what I reserved, and once in a Chevy Tahoe (a three-class upgrade).
Reserving the smallest car group (A), you can expect to drive away in a B/C vehicle - Focus, Civic, Cruze.
Reserving a B group car, you can expect to be put into a C or D group car, and so on.