as a technical guy, i think it's micrometer. there's even an aparatus/ instrument which you could use to measure up to the smallest detail. it's the vernier caliper.
You never stop learning.
I honestly thought that the smallest "imperial" unit was an inch.
I was planning to use it in a quiz that I am setting but now the question will be "In Scotland what kind of geographical feature is an "inch".I think I'll be safer on that one.!!!!!!