I think the best way is to observe reality, see how things and landscapes look  and what's the difference  between vast landscapes and your back garden for example.  Getting as much reference  as possible, studying the landscape photos and books and actually walking outdoor helps a lot.  In Vue scaling the terrain doesn't help too much, you need the details, the atmo effects (haze, fog, aerial perspective) and some objects  from which you get the sense of scale, figures or vehicles  Spedler mentioned above. In your case you try to recreate some kind of Moon  landscape I am not too familiar with and where atmo effects can't really come to use, but as you can see here......
http://www.hq.nasa.gov/alsj/a17/AS17-145-22160HR.jpg

....you need usually several levels of details in you landscape. Rough base and then fill it up with smaller craters, stones....etc......and finally add something what help us to realize how big it is.....( the human tracks here really helps)....
Quite general help but i hope  helpful a little bit....
Ailig