I recently made a lens board with pinhole adapter for a 4x5 Toyo 45AII. The pinhole itself was made in a square of brass shim stock using a standard sewing needle. The problem I had to solve was measuring the pinhole diameter to determine if I was close to an ideal size for the focal length I wanted to use. In this case 75mm. There are different formulae for the ideal size for a given focal length. I won't go into the specifics of calculating the size. In this case I wanted about .34mm.
What made finding the size darn simple was scanning the shim on my flatbed Epson v750, opening the .tff file in Photoshop, and using the ruler tool to measure the diameter. The ruler tool can be found in Photoshop under Image -> Analysis -> Ruler Tool. I scanned the shim at 6400 DPI and was able to use the ruler tool to measure to 1/100th of a millimeter. Being able to see the hole at this magnification also gave me a better view of the roundness of the pinhole.
All in a fast, simple, and probably very accurate way to measure a pinhole. After I completed my measurement, I searched around and found some other folks who had employed a similar techniques. So, there is more info to be found out there too.