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.
Horace, the famous Roman poet, said that the purpose of the poet is to instruct and delight. An epic poem serves both these functions--consider The Odyssey or Beowulf. Moreover, an epic is set against larger-than-life circumstances, while allowing the audience to undertake a journey with the hero. While Star Wars is a trilogy of films, it still exemplifies characteristics of an epic. The epic backdrop and hero's journey are rather self-evident: as the Rebel Alliance battles the Evil Empire across planets and space, the hero Luke Skywalker undergoes each stage of the hero's journey--termed the mono-myth by scholar Joseph Campbell--much like Odysseus. Finally, the trilogy not only provides pleasure to the audience, it also offers instruction. Though the story is simple, it is universal, for what person has not underwent a journey to achieve a goal with the help of friends and mentors. Jungian archetypes permeate Star Wars, but the simple message that good conquers evil and there is a higher spiritual power make it an epic.