Measuring your eye's entry pupil for astronomy is easy if you have a digital camera. In this example, the author used the Nikon Coolpix 2200.

Steps:

1. Go out in the dark and let your eyes adapt for 1-2 hours.
2. Have someone take a picture (with a flash) of your main eye (or aim the camera to your eye and take the picture yourself). Be sure to look at the lens. Here's what the author's eye looks like:
   
3. After you transfer the picture to your computer, open it with Photoshop.
4. Invert the pupil and change the contrast to force the boundary of the iris to stand out.
5. Keeping the Shift key down, draw a horizontal diameter for the iris. Here's what it looks like in my case:
   
6. Change the "Units and Rulers" under "Preferences" to "cm" and "mm".
7. Magnify the image using "ctrl-+" and measure the length of the line against Photoshop's ruler.
8. The author's reads 14 ticks, which divided by two gives ~7mm.