I use a set of metric Allen wrenches. Go out at night and wait for your=20
eyes to be fully dark adapted. At least 20 minutes. Hold an Allen wrench =

one at a time close to your eye and look at a medium bright star. I rest =

the wrench on my cheek. With small sizes you will see two stars, one on=20
each side of the wrench. Work up until you only see one star, on one=20
side of the wrench or the other. I've heard some people can see two=20
stars when using a regular wooden pencil. That's about 7mm. To me a=20
pencil looks like a board! I can just see two stars with a 3mm wrench=20
and only see one with a 4mm thus my pupil is right around 4mm. Sky &=20
Telescope magazine sells a little gauge for measuring nighttime pupils=20
as well. It's a piece of mylar with paired holes from 1mm to 9mm apart.=20
It works on the same principal as my Allen wrench method. You can also=20
use strips of black paper cut to various widths or drill bits.

-Florian
Stargazing.com