1) An extended narrative poem in elevated or dignified language, celebrating the feats of a legendary or traditional hero.
2) A literary or dramatic composition that resembles an extended narrative poem celebrating heroic feats.
3) A series of events considered appropriate to an epic: the epic of the Old West.

The third line above is where interpretation comes in. What is considered "appropriate?"

One film nearly three hours long I have yet to hear referred to as an epic is La Dolce Vita. Most "epics" are historical films by popular definitions. Spartacus, Gone With the Wind, The Ten Commandments, etc.

Two other films that are over three hours long and aren't epics are Altman's Short Cuts and Anderson's Magnolia. I consider Mann's Heat to be a modern crime epic as it's differ is the setting from the other films.

Three other epics that come to mind are all Leone films. Once Upon a Time in America, The Good, The Bad, and The Ugly, and Once Upon a Time in the West.

Now, as for short epics I cannot really think of any, although one that pops in my head is DeMille's The Sign of the Cross.