'Infinate'  terrains are not possible in Vue.  The closest thing is a procedural terrain.  After you have created it and made it's texture;  select the terrain, switch the top view and zoom out until you can see the circle, expand the terrain to the size of the circle.  The circle is the 'horizon'.  

A procedural terrain, or anything this big may slow your computer down.  I wouln't even try to do an ecosystem on it.  You will probably find that this size terrain is useful for high altitude scenes.   I have never used one...yet.

Be sure to take a look at the above mentioned tutorial on the function editor.