The different size aisles are marked in the garage.
Emerald Aisle is first if you enter from the right side (in B terminal at least), it will have vehicles of various sizes, you can pick any size from the Emerald Aisle rows if you have an Emerald Aisle reservation. I have seen full size, SUV's,Vans, Convertibles on the EA at various times (but when I was there last month it was just full size or pickups). If you don't have an Emerald Aisle reservation, then you have to pick from the size row you reserved after visiting the counter (unless you have an Emerald Choice reservation). With an Emerald Aisle reservation, you have the choice of picking from the Emerald Aisle or the size you reserved, so reserve full size, it's usually the same price as intermediate and you are guaranteed at least the full size.