Full quote from reddit user fughp is below, complete with typos and all.
No confirmed info by CDPR, but this GDC presentation contains this interesting slide:
So that’s come 135 square kilometers combined and this is without the No Man’s Land which should presumably be the biggest area in the game.
Said Skyrim is for comparsion estimated to be around 40 sqaure km. Looks like they kinda did not get the original comparsion right.