Well, from what I've seen across the rental industry, the Focus is the compact, the Fusion is the midsize (intermediate), and the Taurus is the fullsize.
On the Thrifty lots I've seen around the country, I usually see the Focus or Dodge Caliber as the compact, the Fusion as the midsize, and the Chevy Malibu or Chevy Impala as the fullsize (I haven't seen too many Tauruses in the Thrifty fleet yet). Standard is kind of an odd car class that's in between midsize and fullsize, but in my experience, if you book a standard, you'll end up with a fullsize, but don't quote me on that.