I read an article about Stephen King's final volume in the Dark Tower series. He's been working on Dark Tower since he was 22 and it is what he calls the world's longest serial novel.
In reality, the record for the longest work of modern fiction is comfortably held by "To-kugawa leyasu" written by Sohachi Yamaoka. To surpass Yamaoka, King would have to write thirty-four more books.
I have my own magnum opus, which I will write in its entirety before I submit it for publication. Will it be seven volumes, or forty? Don't know. It will be as long as the story needs it to be, as every book I write is.
In writing, size only matters when you write for hire, or you go into production and your publisher decides your book is too long to be published profitably. Writing for hire means you write to the publisher's size specifications, and this is harder than it sounds, particularly if your comfort zone is 100K and you're writing 72K on demand. Publishers are now following an unpleasant trend of splitting up long novels into separate volumes, something I'm completely against.
There are writers who think that bigger = better, and if you can't figure out the Freud behind that one, you should toddle back to the convent.
Many aspiring writers believe the more they write, the better writer they become. Practicing your craft is always a great thing, so I don't argue with this philosophy. Just remember that writing a million words a year doesn't automatically entitle you to be published. Nothing does but the quality of your work, and if you're sacrificing quality for quantity, you're doomed to remain unpublished.