I am back, and hope everyone who has gone already, had a good trip. My experience with Alamo was different than what some have described here. I booked a standard and was allowed to choose only from standard. To get a full size, they said I would have to pay more.
Now, in the past I have found across the board with any car rental company, that if they are all out of the car class I had booked, then they will let me get the next available size up, at no extra charge.
But it was not my experience with Alamo that you could get a full size for the same price as a standard, when they have standard cars available.