From what i can remember. Scan the pinhole at at least 1400dpi or larger if possible. Then use the measuring tool that you have in photoshop to get size. If you use the 1400dpi, if my math is right then if 11 dots fit inside the pinhole it is a .25mm hole.
The focal length= a very specific pinhole size. If not the image will be more soft then you may want. A focal length of 40mm needs a pinhole of about .3mm.
When making a pinhole first get a clean soda can and cut out a 1 inch square.
Then find or purchase the smallest needle set you can find. Inset loop end of smallest needle in eraser of pencil(gives you something to hold on to while you rotate the needle).
You will aslo need some 400-600 grit sand paper.
The key to making a good small hole is to alow the needle to make a dimple in the bottom of the metal. Then work the dimple with the sand paper until a hole opens up. Working this way should give you a nice small .2-.3mm hole.
What kind of camera are you making? What type of film?
I hope this helps