At a recent GDC presentation by CD Projekt Red and Umbra Software (called "Solving Visibility and Streaming in The Witcher 3: Wild Hunt with Umbra 3 - catchy title) a particularly interesting fact was revealed (and spotted by Reddit user fughp and later Dualshockers).
Slides from the presentation revealed that the city of Novigrad measures 8.5km x 8.5km (for a total size of 72.25km²) and Skellige is 8km x 8km (meaning a total size of 64km²). Putting that in perspective, the map of Skyrim is estimated to be around 15 square miles, with both The Witcher 3 maps converted to miles and added together totalling 52 square miles. Considering how big Skyrim is/was, that makes Wild Hunt huge.
Still, we've some time left before we'll be able to see just how huge the game is, after it was confirmed last month that Wild Hunt had been delayed until February of next year.