I think some people are using a different definition of "epic song" than I do. http://music.ign.com/articles/781/781987p1.html "The definition of "epic" is as thus: "heroic; majestic; impressively great; of unusually great size or extent."
When one thinks of an epic song a few things come to mind, both based on the aforementioned definition. The first thing that comes to mind is a song featuring rich, poetic storytelling, an epic tale that captures the imagination of all who are privy to its lyrical melody.
The second thing that comes to mind is the length (i.e. the size) of the song. Most standard pop songs conform to a rigid 3-to-4 minute length rule. The truly epic songs tend to breach the 6-minute mark and stretch into that realm of extended bliss with nary a quiver of though in regards to their subsequent dismantling of the standard pop structural guidelines.
However, there is a third aspect to the epic song. This one is far less tangible as it involves the subject of emotion. While great storytelling and extended length are certainly benchmarks of the epic song, there is also the emotional fortitude of said song to take into account. Some songs just feel epic, regardless of lyrical composition or length. They send chills down your spine and generally unfold with a sweeping sense of intensity."