If your machine can make it- you'll have a stunningly vast landscape with a TERRAGEN type of look. But if not- 4K x 4K is still great. The scene above was rendered with a 4K x 4K terrain of Mt. St. Helens and still has lots of "vast space" with some fog and haze tweaked in- like something you might see Terragen.

I bring up Terragen as it plays an important role in getting QUALITY terrain files into Vue and once you know about the Terragen 2 Vue Method- it opens up LOTS of great Terragen files that are on the Net that you can use in Vue.
(Skull Mountain, the 4K Roman City and many other great Mountain landscapes.) BUT this scene you see of Mt. St. Helens did NOT come from a Terragen file, but from a NED BIL file- which I will explain below...