Perspective issues

[Sabonis]

Using perspective projection the farther an object is from the eye, the smaller it appears in the result image.
This relation between distance from the eye and dimension seems to be nonlinear, this amount of nonlinearity depends from glFrustrum parameters?How can I compute this relation?

[Xmas]

With perspective projection the projected size of an object in each dimension is inversely proportional to the distance.

In other words, a 1x1 square at distance 1 and a 4x4 square at distance 4 project to an equal area.