I found a way to have very small rulers.  I am a model railroad nut as well as a dedicated wood worker. I try many times to measure very small spaces.  I was at a hobby shop and bought an inexpensive X-Acto® knife, kept the blade for my other stuff, and got a small stainless steel ruler and cut it into 6 pieces from 1/8", 1/4", 1/2", 3/4", 1", and 2". The X-Acto knife came in a plastic tube which is perfect for keeping it all together.

-Bob Morrison