Why is the movie Star Wars considered an epic?

I would say that Lucas' film can be considered an epic for
a variety of reasons.  The most pressing is that it represents the story of a heroic
quest.  The hero, Skywalker, must defeat an intensely powerful adversary, Darth Vader. 
This is a quest whereby the hero must enlist the help of other such as Han Solo as well
as others to assist him in his goal.  There is an elderly master from whom lessons must
be gained, in the form of Obi- Wan and Yoda, and understanding must be derived.  At the
same time, while there is an external quest in order to gain a sense of justice in the
world, the quest is also internal, as Skywalker must understand more about himself and
recognize that the balance through which the world must be restored is as much personal
and as it is external.  In this, another quality of the epic is met in that both
subjective and external journeys are facilitated.  At the same time, Lucas' film can be
seen as an epic because it presents these voyages on a scale that is meant to overwhelm
the viewer, as the "other dimension" of space and intergalactic warfare helps to enhance
the epic proportions of the work.