Scale can be a tricky thing in a painting. It is not important to paint large paintings to have a sense of grandeur on your art. What is important is making yourself aware of the principles and using those to your advantage so you don’t have to resort to clichés of constantly adding tiny people or other recognizable objects to make the illusion work. While these are one way of creating a sense of scale their use should never be a substitution for good design and a thoughtful approach to the subject at hand.
I think the first thing you need to create a sense of scale is correct aerial perspective. All your vanishing points must be accurate and your horizon line must be established. This is true for landscapes without manmade structures as well as paintings with them. Perspective will create overlapping forms and the proper arrangement of these will help the illusion of space and distance.
A secondary effect of scale is atmosphere; painting things with the proper lessening of chroma and value as they recede into the distance. There is no formula for this and you must be a keen observer of the subtle shifts that take place and act as visual clues for scale and distance.
Another effect is the loss of detail as you take in larger areas of view. It seems counter intuitive at first but the bigger something is the less detail you can see on it. If you can see a complete eight story building in your view, the inclusion of individual panes of glass on the windows only shrinks the idea of scale for the viewer.
It is the same with natural things in the landscape a distant hillside looks smaller if you paint every tree on it as opposed to getting its overall form and color and value. People often do this with large bodies of water. They focus on the waves and when they paint them the waves height to width ratio is enormously exaggerated reducing the scale of the ocean in the process.
Lighting is also important for a sense of scale. Outdoors light falls in parallel rays and you have to make sure you paint it that way or you will give the illusion of indoor point source lighting and shrink the sense of scale.
Many times the emotional response we feel to a subject is based in large part on a sweeping sense of scale. Making yourself aware of the effect scale has on what you are observing will help you capture that sense of scale in even small paintings
