Whenever you're drawing an environment or a layout, it can be easy to forget to give the viewer a sense of scale. If your drawing or painting doesn't contain any people, animals, or other objects that have a consistent size, it can be very confusing as to how big or small the spaces and elements are in your artwork.
This is the Great Hall in the Palace of Westminster in London. In this photo, it's very hard to tell just how big the space is.
For several reasons, this isn't the best example...after all, when we see a photo like this we usually assume it was taken at typical human height (somewhere between 5 or 6 feet) so it's not all that ambiguous. But hopefully you get my overall point. It's hard to tell from the photo above just how big or small this space is.
The problem can be compounded by the fact that many times when artists paint imaginary scenes to explore how an imaginary place might look, they put the camera up high so we can see more of the landscape. This can add further confusion because we don't have the vantage point of being about 5 - 6 feet off the ground (which is how we normally view the world every day, of course). Once we're up high looking down on a landscape it can start to feel like we're looking down at a toy landscape or a model train layout or something. Scale gets even more muddled.
So here are two historic paintings of the Great Hall at the Palace of Westminster. Compare them to the photo above (which lacks people or any other scale clue). Notice how the addition of people instantly gives the space context. You immediately know how large the space is because we all know exactly how tall people are so they make a great yardstick for our eyes.
Didn't the space look smaller in the photo at top?
So really this is just a reminder to always include some type of object to give the viewer a sense of scale. It's a simple thing...but very easy to forget. And the more fantastic and otherworldly the landscape, the more important it is.
Many things come in a standard shape and size that can help to give a painting scale; people (obviously), animals, cars, houses, airplanes, boats (anything from sailboats to cruise ships), trees, etc.
Sometime I'll take some paintings and Photoshop out the elements that give it scale to illustrate this further...but right now hopefully you find this reminder helpful!