The car ID is scanned or written down by the clerk at the lot exit. If you arbitrarily take a car that is different from the kind you booked, your contract and invoice might be refigured at a higher rate when you return the car. But the rental agency should have a provision to give you a car that holds the persons and baggage that you booked a car for, without extra charge or large delay.
Larger cars consume more gas per mile. You already knew that.
Some not so commonly rented vehicles, notably convertibles and pickup trucks, might not be covered by your credit card rental collision benefit.