How big? Well at Dual Shockers someone did the calculations. Based on information delivered at a GDC talk super. Three times bigger than Skyrim big. That's pretty big.
To begin with, I'd be taking these numbers with a grain of salt. The calculations were made based on two areas referenced in a talk: one area was said to be 8km by 8km, another 8.5km by 8.5km. When you combine the two, say Dual Shockers, that adds up to an area of 52 square miles, more than three times the 14.8 square miles of Skyrim's landmass.
I have no doubt these numbers are legit, I just tend to have a little bit of doubt regarding numbers like these and how they actually impact the experience itself. It's a trick Rockstar pulls with every game it releases. Grand Theft Auto X is at least four times as big as Grand Theft Auto VII and VIII combined! I don't know if it ever really changes things that much. Because let's face facts: Skyrim was bloody big. It was as big as it needed to be, and I suspect that The Witcher 3 will also be as big as it needs to be.
And if these numbers have any shred of truth about them, The Witcher 3 is going to be really, really big.