As other's have said, emerald reserve means you pay for only that size car. The perk is that you bypass the counter and your rental is prepaid. So if you book full size, then you go to the full size lane and choose any full size car you want.
however, emerald aisle, means you book and pay for a mid-size car, and then go to the emerald aisle section and choose anything in the emerald aisle lane. It could be bigger cars, but it could also be only mid-size cars. You have to be ok with a mid-size car when you choose this option. Yes, it does happen and I've been many times when it only has a few cars there and they are mid-size. My daughter ended up with a mid-size car from the EA lane.
Then there is the executive aisle and that is for folks like me, who earned that level by booking 40 or more days a year the previous year. I can choose from either the Executive aisle or the Emerald aisle.
Also if you're flying into MCO there are TWO sides of the airport, Side A and Side B. Generally most folks go to the side where their luggage comes out, but the rental car companies are on both sides. A few times I did not like the choices on Side A after getting my luggage, so I went up the elevators and over to the Side B side to see what the choices were there. The choices can be different, and sometimes worse.
Good Luck! We've been Emerald Club members for 10 years now and love it!