Don Quixote is a favourite of mine JD!!
Um although I agree it's more about quality, I generally wouln't refer to a really good book as epic unless it has a bit of weight, Perchance thats just me and my lack of thought/education.
No, I think it's just a case of (as with so many other things) we no longer understand the roots of terms, and therefore we come to have a misconception of what those words mean. For example, the earliest epic -- Gilgamesh -- is anything but long... at least as we have it now. Yet an epic it most definitely is. Cat's Cradle is about the end of the world... again, handled in a humorous vein, so it's a "comic epic"... funny, but based (as all great humor is) on tragedy. Leiber's Fafhrd and Gray Mouser tales become epic, as they sprawl all around their world (and into ours), they go up against all sorts of things (including gods from both worlds), and they blend heroism, humor, tragedy, deep insight, and all the other elements of true epic in them... though the heroes are by no means
sans peur et sans reproche.... At any rate, none of those books were particularly lengthy (at least, as Leiber published them originally).
But we've come so strongly to associate "epic" with "large, sprawling" (in the sense of heft) that I'm not sure we can get away from that one -- at least not without considerable effort.
Should we? Well, my feeling is that we shouldn't see epic as
necessarily lengthy, though it
may be so; otherwise we do something of a disservice to both types of (genuine) epic tales.
(We also run the risk -- as we are seeing today -- of associating "epic" with "length" in another fashion... if it is one of these doorstops, then it's "epic"... whether it be golden or pure crap.....)