Deriving blur from real optical formulae
First we can calculate the physical diameter of CoC in the image plane, given the lens parameters. This equation is from Wikipedia – Circle of confusion:
$$
c = {|S_2 - S_1| \over S_2} {f^2 \over N(S_1 - f)}
$$
where the variables are:


$c$: the physical CoC diameter in the image plane
$S_1$: focal distance (the distance at which a subject would be in perfect focus)
$S_2$: actual distance to the subject (the point whose CoC we are calculating)
$f$: the focal length of the lens
$N$: the f-number at which the lens aperture is set.


Then, divide this CoC diameter by the film or sensor frame size to get the blur diameter in texture coordinates. You can additionally multiply it by your rendering resolution to get the blur diameter in pixels if desired.

As an example calculation, suppose we render a 1920×1080 image, as if through a 85mm lens at f/1.4, on a 35mm camera (frame size 36×24 mm), and focused at 3m while our subject is 5m away. Then we can calculate:
$$
{|5\text{m} - 3\text{m}| \over 5\text{m}} {(85\text{mm})^2 \over 1.4(3\text{m} - 85\text{mm})} {1920\text{px} \over 36\text{mm}}
$$
(Don't forget to convert units, since we are mixing meters and millimeters here!) I get a blur diameter of ~38 px, so the radius would therefore be ~19 px.