I know Insiders get a free upgrade, but I don't know if there is an upgrade past a full size.
The way you get the upgrade is to use the coupon code for the upgrade when you make the reservation. So, you should make a reservation for a midsize, using the upgrade code. You will see that the reservation is for a standard vehicle, even though it is the midsize price. I say to do midsize because, as others have mentioned, at MCO, there is no standard aisle. You will get to pick from the full size aisle.