I am a bit confused as to what constitutes as a Standard Size Car vs the Midsize and Fullsize car? Is a Standard Size car as big as a full size car in the backseat , truck space, front seat etc or like a Midsize Car? For instance they consider a Toyota Corolla as a Midsize Car when Toyota calls it a compact car unless that changed.
If I had a car rental agency I would hold true to form so when you book a Full Size car you are guaranteed a Ford Taurus or a Midsize Car gets you a Ford Fusion etc. Its like reserving a Suite and only getting the Junior Suite at a hotel.
You're dealing with Budget rent a car. Lower your expectations, and then lower them another notch. Everytime you think you can rely on "something" (miles, promotion, etc.) they change the rulles, without telling you (or sometimes their non-corporate locations). So if you think you're goin' get them to adhere to some definition as to what constitutes a certain size of car, you're in dream land.
Programs: US Platinum, United 1k, HHonors Diamond, Avis CC, Priority Club Platinum, Marriott Gold
From the Budget.com car guide:
Intermediate: 3 briefcases, 2 small bags, 1 large bag. Seats 5
Standard: 2 briefcases, 2 small bags, 2 large bags. Seats 5
Full Size: 2 briefcases, 2 small bags, 2 large bags. Seats 6
I have a pretty comprehensive list of cars in each category over on the Avis page (Avis & Budget now have a shared fleet). However, from my experience, what's listed on their car guide pages are just that... guides, but not definitive (I once reserved a Premium car because of bad weather and wanted ABS, which was listed as standard for that class. I got a 300 without ABS & was told by the local office AND customer service that even though it's listed under "standard amenities" on the Avis page, it's only a guide, so too bad).
Also, Avis categorizes many cars as full size even though they only seat 5 (Taurus, Sonata, 2009 Malibu, Aura, Camry, Altima, Galant, etc etc). Personally, I think they should be held to US Gov't rankings for their categories, but I don't see it ever happening.
I never noticed it said a fullsize seats 6 until you mentioned it. I wonder what would happen if you showed up with 5 other people and said you booked fullsize because it seated 6. Would they upgrade you to a minivan? Or maybe it would end up like your Premium w/o ABS situation. I've also noticed the equipment list is preceded with "Most _____ rental cars include", so they can technically say your Luxury car with roll-up windows and manual door locks wasn't one of the "most".