In Orlando, you'll also find economy and compacts there, too. MCO is one of the few EA locations that's set up to handle all car classes for EA reservations. If you book a compact online, your confirmation will say counter but you can just go right out to the EA and pick your own car as you would the bigger cars, then drive to the checkout booth, show your license, EA card, and any coupons you have and off you go.
I don't know what they'll do if you reserve an ecomomy or a comapct and then try to take a full size. Somehow I don't see that scenario working as the bigger cars do.