Factors affecting the size of a shadow

What factors affects the size of a shadow and how would you derive the diameter of a shadow of a circular object on a flat screen?

-
It's based on the distance and orientation of the light source, whether it becomes an ellipse or a larger circle ;-) –  Waffle's Crazy Peanut May 30 '13 at 13:19
If the light source isn't pointlike, the shadow doesn't have sharp edges. –  Ben Crowell May 30 '13 at 15:36

Instead of the Circular object I use a cardboard with a circular cavity that will behave same as the object, instead the lighted portion on screen would be shadow in that case :)
$$\tan\theta=\dfrac r d =\dfrac R{d+D}$$