When I've made pinholes, I've used very small sewing needles (quilting ones are labeled by size) and not gone all the way through. I then smooth the area from each side using emery cloth (cloth based sandpaper intended for metal). I use pieces of soda cans as my material. I usually color both sides with either spray paint or Sharpie marker. To measure, I have a magnifier with a scale built in, but a magnifying glass and a metric ruler should work fine. Mine are usually around 0.5mm - 0.6mm, though I can make smaller ones if I try hard.