With National, u can reserve mid size and take any car in the exec and exec-elite lot which has good chance of having one of the big guys (IME). Avis pres entitles you to double upgrade, but many times you can do much better, so I don't think it's worth booking higher than full size (I say full and not lower bc usually there isn't a big price difference from compact up to full size, and I don't think they will let you get away with compact res and driving out with big SUV, though I'm sure that has happened).