Known size or scale is a depth cue that reveals depth through associations of figures and shapes that have an intrinsic size. A viewer knows that a house is larger than a mosquito so that if the two were approximately the same scale in an image, the house would be perceived as further away.

Even though the two figures are approximately the same size, the assumption is made that the duck is closer to the viewer than the elephant (as opposed to a giant duck or a miniature elephant).

Known size of a wave, boats, and the mountain - the assumption is that the mountain is far away rather than a tiny mountain.
Katsushika Hokusai, The Great Wave Off Kanagawa. 1823-29, color wookcut.

Known size of a people - the assumption is that the child (indicated in red) is further away rather than a tiny person floating in the air directly in front of the two adults (indicated in yellow).
Georges Seurat, A Sunday Afternoon on the Island of La Grande Jatte. 1884-1886, oil on canvas.