The slides mention the size of two of the areas included in the game: the city of Novigrad measures 8.5 x 8.5 km for a total size of 72,25 square km, while Skellige is 8 x 8 km, meaning 64 square km. Summed up, the two areas cover 136,25 square km, which convert to just north of 52 square miles.
In essence, just those two areas are roughly 3.5 times the size of the entirety of Skyrim's map. Last year, CD Projekt Red had mentioned that The Witcher 3 was already 20% larger than Skyrim, so it may be safe to say that the game has certainly grown in scope since that initial reveal.
(via DualShockers)