Here's what I did:

1) Stand in front of mirror with measuring stick and get a read on the
outside diameter of my iris.
2) Take a picture of my eye in a darkened room using the digital camera set
to basic flash mode (no double flashing for light sense. Didn't worry too
much about focus, and took a couple pictures at varying distances to get a
usable focused image.)
3) Transfer image to laptop via USB.
4) Display image and hold measuring stick to displayed image of eye, iris
and pupil.
5) Measure the diameter of the iris in the picture.
6) Measure the diameter of the pupil in the picture.
7) Calculate the percentage of pupil occupying the iris, by diameter.
8) Multiply the mirror and measuring stick value by percentage derived in
step 7.

Result:
Iris diameter = 13mm
"Dark" pupil diameter by percentage = 50%
Dark pupil diameter = 6.5mm

I'm 44 years old.

-Stephen Paul