According to the band, they have six epics: A Change of Seasons, Six Degrees of Inner Turbulence, Octavarium, In the Presence of Enemies, The Count of Tuscany, and Illumination Theory. On these forums, a lot of people don't consider TCoT an epic and a small minority consider A Mind Beside Itself to be an epic instead of a suite. I have also seen posts claiming that The Twelve-Step Suite is an epic but those are very rare. Really anything shorter than TCoT but over 10 minutes (along with the 9:31 Metropolis) is usually called a mini-epic.
I like the term mini-epic to represent certain songs. Perhaps ones like ITNOG, ToT, BAI, Scarred, The Killing Hand. Problem is it still gets out of control because DT still have a lot of long songs that some wouldn't class as 'mini epic' either. Oh God I'm getting into it again, sorry.