You are not guaranteed an upgrade, but you are guaranteed the size car you reserved. For example, if you reserve a full size and none are on the aisle, you can get one from the full size aisle (see the attendant).
When I was there last month, they had a variety of cars to pick from, full size sedans, convertibles, SUV's. I got a 2001 Toyota Camry with 228 miles on it.