World Compositor - too small, can`t zoom in

Could someone give me some light? Having an annoying problem here.
The map appears way too small inside world compositor, and I cant zoom in after a certain point. So it shows me a little square inside a huge square area. Weird, because no matter what size I make my terrain, it appears as a tiny square inside the world compositor and I cant zoom in more than that.

My bad, it was actually about resizing the terrain.
However, still I have a question.
For an open world with good performance, what size would be best for each square piece?