I've worked a bit on the geometric optics that describes out of focus disks. The great data given here allowed me to check the theory predictions. Things agree reasonably well.

Theory predicts the disk diameter increases in proportion the actual aperture diameter and to distance in front of or behind the in-focus subject. This is countered by the perspective effect which makes the disk apparently decrease in size as the point creating it gets farther behind the subject.

Here's the data in graphics form:

In the equation, So is the camera-subject distance and S is the camera-background distance; Mo is the optical magnification. The Focal_length/F-Number term is equal to the actual aperture opening so in a sense, the equation is independent of focal length (it cancels in the ratio so all that's left is the aperture size.)

Notice that the out-of-focus disk diameter approaches the actual aperture size times the magnification as the point approaches infinity.

Dave