For some reason my brain just got stuck on this a while back and I had to go down the rabbit hole. Marcel Proust's 'In Search of Lost Time' is the one that always gets mentioned, and rightfully so—it's a monster of introspection and social observation. But the actual word count varies wildly by translation; the English version by C.K. Scott Moncrief and Terence Kilmartin is often cited around 1.2 million. That's the one I attempted in college and made it through maybe two volumes before life got in the way.
Then you've got things like 'Artamène ou le Grand Cyrus', a 17th-century French romance that supposedly tops 2 million words. I've never met anyone who's actually read it, and I'm not sure I want to. It feels more like a historical curiosity than something with a living readership. The real modern contenders are online serials. 'The Wandering Inn' by pirateaba has to be up there, probably past 12 million words at this point and still updating. It's a different beast entirely—consumed in chunks over years, not as a single physical object. That distinction matters more to me than the raw numbers; the experience of reading something that long and ongoing is completely unlike tackling a classic doorstopper.