The true epic contains many different characteristics or qualities. Epics, in order to be deemed so, must include the following characteristics. (Characteristics are bolded and the examples which prove Beowulf to be an epic follow the characteristic.)
Characters possesses legendary or historical significance and/or national importance.
Beowulf, the character, possesses many of the characteristics the Anglo-Saxon culture idealized. He was strong, generous, spoke well (with elevated language), and possessed a bravery which surpassed all other characters in the text.
The settings of epics are vast.
The setting of Beowulf is immense. Beowulf travels from his homelands, the Geatlands (Sweden) to Hrothgar's Danelands (Denmark). Not only does he travel, the text tells of his swimming challenge with Brecca (across the sea) and his descent into the "underworld" (Grendel's mother's lair).
The actions of the hero are courageous.
Beowulf's actions prove to be very courageous. He fights Grendel (with no weapons) after no others are able to defeat him. Not only that, he takes on the challenge of fighting Grendel's mother (after she takes the life of Hrothgar's right-hand-man). These acts ask for no one but the most courageous.
Supernatural forces act on behalf of (or intervene for) the hero.
While the original text was Pagan, many readers and students tend to read the Christian perspective version. In this version, Beowulf never fears any battle. He knows that his fate has been deemed by God already. ("Let the wise God, the holy Lord, decree success on whichever side seems right to Him!” Chapter 10.) Later, when Beowulf is fighting Grendel's mother, and Hrunting has failed him, Beowulf finds an ancient giant's sword. One could assume that God has provided Beowulf with the weapon (showing supernatural intervention).