Answer :

OBVIOUSLY ITS THE BACTERIA THAT IS THE SMALLEST
2MICROMETER THICK AND 0.5 MICROMETER WIDE
smallest mammals are Bumble bee bat and Etruscan shrew.
smallest fishes are those belong to the carp species.
smallest bird is the bee humming bird
smallest insect is fairy fly
smallest reptile is dwarf gecko
smallest amphibian is Brazilian gold frog

overall the smallest is Kingdom animal is a microscopic water dwelling invertebrate.