Its difficult to compare, but I felt that the gameworld of Horizon was about the same size as The Witcher 3 (minus Skellige).
And yes you can walk and walk in Horizon, same as in The Witcher 3. Difference is that you will bump into large robots here so except for some 'friendlier' areas you are also sneaking around alot because you don't want to fight a gigantic robot every 15 minutes.
That's the fun thing actually, a lot of the bigger robots arent even part of the story missions, you just bump into them. You can also choose to sneak past them, especially in the beginning you are way too scared to fight them. Then when you get better in the game you want to try it out. These fights can go on for 10, 15 minutes.
It's not a problem that you can sneak past them, it is actually more engrossing this way, it feels realistic.
But about the gameworld, its very varied. Maybe even a bit too varied as the weather/vegetation can switch pretty quickly from area to area, haha, but Im not really serious in calling that a negative point or anything. But compared to the Witcher, which had like, the same weather/temperature kind of area's in the main game world, Horizon is more varied in that regard.