How to Place People in Perspective in an Architectural or Landscape Drawing
Placing figures of various heights to create a sense of perspective is easy if you know the trick. Simply place the eyes of each figure at the same height as the horizon line, with adjustments for figures that are shorter or taller than the most foreground figure, or are performing activities that would elevate or lower them.
The reason is because your brain interprets the vertical location of the viewer at your own eye level, so an infinite plane at eye level cuts through the eye height of all figures the same height as the assumed viewer, and extends out to the infinite horizon.
Figures furthermore can have declining detail as they shrink in size, which mirrors our own perception of detail in near and distant objects.
Here's an example: