I use a Lincoln Memorial penny for scale.

By duplicating the background layer, then dragging Layer 1 over the subject (at 50% opacity) you can measure tiny subjects.

Each pillar on the Lincoln Memorial is roughly 1 mm. Or on 1 mm centers, you might say--or from the beginning of one pillar to the beginning of the next. Makes a good, inexpensive scale that you can afford to lose and easily replace. You probably have one in your pocket right now!

It's close enough for my purposes. :-)