1. Get the book by Jim Shull entitled "The Hole Thing". What a great book! Your library can inerlibrary loan a copy for you if they don't have it. I sometimes see them on bookfinder.com and on ebay.com.
2. If you cannot get the book because like me you are too tired cold or scared to do so, then get ye to the nearest fabric store and buy an assortment of hand sewing needles. Measure the needle with a micrometer or go to any machine shop where they will happily measure the needles for you and write the diameter on the handy masking tape tabs you have already attached to the needles so they don't have to.
3. Push the desired needle eye first into a pencil eraser which is still attached to the pencil. It makes them muche easier to handle that way. Try to make it coaxial with the pencil which is now the handle for the needle.
4. Put a piece of aluminum foil onto a piece of cardstock (Cap'n Crunch boxes are made from the industries finest card stock). Press the neddle into the foil while spinning the handle (pencil). This will make a very nice and perfectly round hole of the diameter you desire. If you believe this statement skip step 5 and imediately begin making breathtaking opinhole images.
5. Oh ye of little faith. If you just HAVE to measure the pinhole, it can be done using a microscope. Focus the microscope on your calibrated needle. Tape a piece of velum (special drawing paper made from sheep intestines or something) over the eyepiece of a standard light microscope. mark the shadow cast on the velum by the needle. replace the needle with the pinhole and voila! you can compare the diameter of the calibrated needle to the diameter of the pinhole. Clever huh?