The Witcher 3 is a very big game, with over 200 hours of content. That's why it unsurprisingly has a very big script to accompany it. Games writing has a lot of variables too, so as you explore new dialogue options, that is only more dialogue and actions that were written that you didn't explore.
But how big exactly is it? Well, according to IGN, who found the numbers on Side's site, the company who handled the voice acting for the game, the games script was around 450,000 words long. As IGN note, the average novel is around 80,000 to 100,000 words.
That's somewhere in the region of 4-6 whole novels which should give you an idea at what a huge undertaking this game was.
This won't account for the frankly huge amount of flavour text you will see in the world either, be it item descriptions or just the notes you pick up among the world. This game is a monumental achievement in these terms, and the fact that the writing is consistently good to great is even more staggering.