I am sorry to hear of your problems, there have been a number of complaints about Advantage but you are best trying to get a result with Argus who took your money. Have you looked at other websites and checked that everyone else, Hertz, Avis and Budget for example would agree that the car you received was not the same size as the one booked?
Has Advanatge ever admitted you didn't get what you paid for, presumably you didn't complain on the spot or when you returned the car? The problem may be that fact that you signed a contract with Advantage when you picked up the car may have a term which says you are happy with the car you have recieved and they may use that to ignore your problem. If the car was much smaller than you booked, a sub compact instead of a full size it should have been obvious, what did you book and what did you get?