When you make your reservation through Emerald Club, they will tell you to go directly to the Emerald Aisle and select the car of your choice. Emerald aisle is only available for intermediate, standard and full sized cars, any other car is either choice or counter service. If you don't find a car you like on the EA, you can select from the aisle corresponding to the type of car you rented. IE: if there is nothing on the aisle that you like and you have reserved a full sized car you can go to the full size aisle and select one from there. Occasionally there are premium, luxury, suv, minivans, or converts on the aisle. BUT, don't make a reservation for a full size in the hopes of getting a van or something that would not normally be there, this is an occasional thing and something I would not bank on.
Usually you will find that booking an intermediate is the same price as a full sized car, so, go for the full size car.
I hope all this has made sense and please, don't hesitate to ask more questions if you need to. It is a nice way to rent a car and I have always been satisfied with the cars I have rented this way.