Mosr rental counters try to hard-sell you an upgrade, I don't know of any that don't. When you get there, before they launch into their spill, just say something like "I'm so glad i booked through you guys, you were the only ones who had the size i want available - i wouldn't feel safe driving anything bigger..." and it will put them off a bit, maybe entirely!!!
It's on the arrivals level of MCO and then you walk over to the garage to choose your car.