With the latest changes to Emerald Aisle you may book an intermediate and you will get an intermediate or possibly better. If you require a full size, a mini van, SUV, etc it is best to reserve that specific type. Previously with EA you could reserve with intermediate or full size and if no full size were available you could take one from the full size aisle.
At the airport you proceed to the garage and take the car of your choice from EA. As stated previously you are guaranteed an intermediate, but other upgrades may be available. You head (in car) to the gate and they process your paperwork and you are off.