From AmtWiki


The following rules are from an out of date rule-set A Monster Vulnerability.
From the Book of Engoku:


There[sic] creatures are too small to see (unless VERY close, within 3 feet), cannot move about the field under their own power without Flight, and cannot take part in combat. Anybody larger than Short can Trample them (assuming they can find them).

Currently, there are no known monsters intrinsically bearing this Vulnerability, but monsters with Change Size, such as Konaki Jiji, can choose to bear it using that ability.