The Divine Comedy is often considered an epic. It is common to associate epics with larger-than-life heroes and grand illustrations of warfare (think of Homer's works), making The Divine Comedy an atypical example of the genre.
For one thing, Dante is a rather passive hero figure. He has to be guided through the afterlife by Virgil and then Beatrice, and he has no control over what he is shown. However, The Divine Comedy's story takes the form of a long journey and it begins in medias res (in the middle of the action), both common devices in epic literature.
What makes Dante's poem a true epic is its depiction of the struggle between good and evil, which is another common theme in world epics. Dante witnesses the depravities of sin and the redeeming qualities of divine love. The souls he encounters during his journey teach him about the nature of humankind and God, giving him the tools to live well once he returns to his mortal life on earth. While he does not fight physical battles, he experiences an inner transformation, making him into a different man by the epic's conclusion.
The massive settings of The Divine Comedy also qualify it for epic status. The vistas of Hell, Purgatory, and Heaven are described in impressive detail.