As an Emerald Club member, you are entitled to pick any car off the Emerald Aisle on a mid-size reservation. However, it seems like you want a bigger car, with your Premium reservation. It is very unlikely to find a Premium or better car in the Emerald Aisle. If your status was Executive (or Executive Elite), it is quite possible to find a Premium (or even Luxury in rare cases) in the Executive Selection (but you are not entitled to use this section until you hit 12 rental credits in one year).