Tbh if I was in the devs shoes I wouldn't be sitting there counting the size of the world and comparing it with past games. I would be focusing on working on the game in question, they still have a lot to polish and the deadline is looming. I am doubtful they actually know a precise figure for how big the world actually is.
How can you act so naive...Of-course they know the size of the world, they knew the size of the terrain before they put anything on it.
If you watch the interviews you can notice that they pre-prepared answers for questions that would come up, and the answer for the world size was 35x The Witcher 2.
They don't want to give exact numbers as I said. If the gameworld ix 8x8 km that's less than GTA V maybe they don't want people comparing them to that. This fits with the 20% bigger than Skyrim thing we heard before.
Or maybe they don't want to give exact numbers because if that GDC slide was true and only Skelige Isles were 8.0 x8.0 and Novigrad&Outskirts was 8.5x8.5 then the world is so massive that they want to keep this information under wraps for future reveal and future hype.