I'm referring to the Italian version's running length.

The Good, The Bad, and the Ugly fits the definition perfectly. Brilliant cinematography, Morricone score, a film of an antihero as well as two other men in a story of greed. A huge film that fits the definition the same way Gone With the Wind would. It may not be a historical film, but that's not what an epic is.

To be honest, it's hard for me to describe why, I think of "epic" as a feeling, or state of mind if you will of a film. I can't explain why Short Cuts is not an epic and Gone With the Wind is.

Here is pretty much a feeling of what I think it is.

REFERENCE
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Epic Films often take an historical or imagined event, mythic, legendary, or heroic figure, and add an extravagant setting and lavish costumes, accompanied by grandeur and spectacle and a sweeping musical score. Epics, costume dramas, historical dramas, war film epics, medieval romps, or 'period pictures' are tales that often cover a large expanse of time set against a vast, panoramic backdrop. In an episodic manner, they follow the continuing adventures of the hero(s), who are presented in the context of great historical events of the past.

Epics are historical films that recreate past events. They are expensive and lavish to produce, because they require elaborate and panoramic settings, on-location filming, authentic period costumes, inflated action on a massive scale and large casts of characters. Biopic (biographical) films are often less lavish versions of the epic film.

Epics often rewrite history, suffering from inauthenticity, fictitious recreations, excessive religiosity, hard-to-follow details and characters, romantic dreamworlds, ostentatious vulgarity, political correctness, and leaden scripts. Accuracy is sometimes sacrificed: the chronology is telescoped or modified, and the political/historical forces take a back seat to the personalization and ideological slant of the story (i.e., the 'poetic license' of Oliver Stone's controversial JFK (1991) immediately comes to mind).