In practice, any number greater than 100k, or less than
0.001 in pov is asking for floating point problems.

The apparent scaling of an object varies linearly in
relation to the camera.
So if you move an object along a vector from
the actual location toward the camera to 10% of its
actual distance, the object should be 10% of its
actual size to look the same. In other words if
the ratio of the radius to the distance is the same
the object will appear the same size.

Here's a sample...
camera {
   location <0,0,-4>
   direction 1.5*z
   right     x*image_width/image_height
   look_at   <0.0, 0.0,  0.0>
}
sphere {<0,0,0>, 1
   texture {
      pigment{rgb <1,0,0>}
      finish{ambient 1}
   }
   #local SC = 0.5; //should look the same no matter what number SC is
   scale SC
   translate SC*4*z-4*z
}

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