Measuring your eye's entry pupil for astronomy is easy if you have a digital camera. In this example, I've 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 mine 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) Mine reads 14 ticks, which divided by two gives ~7mm.