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.