You can just go online and start the process of reserving a car and it will show you what types of vehicles are typically provided for full-size or premium. If the
Alamo site doesn't show it, go to a travel service like Expedia and try there.
I'd actually use Expedia because you can see many companies' offerings. You may find that another company offers a better car for the category you are interested in.
The MIA Rental Car Center is a very large facility and you should not get any surprises.