Yes you can. This is known as "self-downgrading" (as opposed to a rental agent doing it). When you reserve a Fullsize as an Executive, you pay the Midsize price, and get access to the Executive Selection Aisle. The Executive Selection aisle features Fullsize and higher class cars.

If you don't like the selection there, you are permitted to select a car off the Emerald Aisle, which as I'm sure you know features Midsize and Standard class cars, with the occasional higher class cars.

If you're still not finding anything you like, and you're at a location that also has Emerald Reserve (which I believe all ES/EA locations do), you can pick a car from the Fullsize row, or any class below that. I've done this at YYZ many times when the Executive row is nothing exciting but the Midsize section has a loaded Focus, for example.

If the EA/ES is nearly empty, or if it's "pick your color of Malibu" day, you can also find a roving greeter to see about getting an upgrade, say to a Premium car. However, this varies by location, demand, time of day, etc. I've generally found that by making myself known on a friendly basis at locations I frequent means they're usually willing to keep me happy if it's a slow day.

Best of luck!