For some authors, it isn't enough to write a book. Some authors aspire to write the great American novel. That usually means it's an epic novel. And when I say epic, I'm referring to the length of the story. In books, epic is a sort of grandiose way of saying super long.
Some stories just can't help but to be big. Gone With the Wind, my favorite, is a massive book. Yet the story simply can't be shortened, and that's just how it is. Despite the length of the novel, I wouldn't want to spend one less moment with Scarlett.
But a book doesn't automatically have merit simply because it's long. Extreme length is a huge, huge turn-off to many readers, in fact. An epic novel is epic because it usually spans many, many years and details many different events. There's a lot of change and character growth, many obstacles and challenges...a boatload of characters and interaction. A lot happens, and that keeps readers engaged.
Writing a long book doesn't make it epic, and regular readers of the blog will know that I'm an advocate for brevity. If you can say it in fewer words, do it. Readers don't want to get bogged down in lengthy descriptions of the way leaves blow in the wind, for example. In today's world of ereading, the epic novel could become the next literary dinosaur. So write lengthy novels cautiously and with a keen eye for details you can gloss right over.
Read your story, and ask yourself if it's really an epic...or just too damn long. If you don't want to read your story, readers won't want to do so, either.