You have two options available.
1. PC members are supposed to get a gauranteed one-class upgrade for gasoline-powered sedans with a maximum upgrade to a full-size vehicle (class F) at participating locations (DEN should be one). Any upgrades for vehicles outside of the above are exceptions and don't happen that often.
2. If you rent a midsize vehicle or larger, you get access to DEN's Ultimate Choice lot.
Consolidated "Renting at DEN (DIA); What Kind of Cars to Expect?" Thread