This page shows a number of depth cues in 3d rendering separately. The images may seem weird since a realistic image contains all cues.
Size is one of the two strong clues.
Obscurance is the second strong clue.
With 3D glasses or a video-wall the participant will focus at infinity and expect nearer objects to be out of focus. On a desktop monitor the opposite is the case.
Fog makes objects further away less saturated.
Objects nearer to the viewer have a larger disparity
These images show the use of three depth clues. Light angle, shadow and light intensity.
Combining all clues but fog and disparity