As a Presidents club member, I would think that if you don't like the car you're assigned, they'll gladly change it for you. They always seem to accomodate me, and i'm only First.
If you start reserving compact, I would be willing to bet that you'd still get upgraded to fullsize or premium (I'm guessing). So you'll likely have to go in to request a smaller car anyway.
My opinion/suggestion is that you reserve the compact.^