pinhead

The moment the nitre was red hot, the coal, previously reduced to small pieces of the size of a pinhead, was projected in portions of one or two grains at a time…

I don't want to be a pinhead no more / I just met a nurse that I could go for

The microcephalic idiot is an unfortunate with a "pinhead," sometimes exhibited as a "what's-it" in circus side-shows, whose mental age never goes beyond that of an imbecile.

A newly hatched gecko will need pretty small crickets, but you will not have to go all the way down to pinheads.