Let's put it this way, the rental vehicles in each class can vary by branch based on local fleet deals and a variety of other factors. A Maxima in a low trim might be a Full Size whereas an Accord with leather interior might be a Premium. A Corolla might be rented as a Standard Size with its usual getup but the version with no automatic locks and roller windows ... would probably still be rented as a Standard
My local branch happily substitutes anything in the "expensive" range for "expensive" reservations, so when I reserve a Premium I might end up with Luxury Car, SUV, or Pickup Truck. I've also had a Lincoln MKS, Cadillac CTS, Chrysler 300 (usually Full Size but nice trim, etc.) and similar. Since they've now split the class into Premium and Premium Elite, however, you should probably expect something more Maxima-like.