SIZISTZOO

Largest and smallest creatures:

world's largest moth - Atlas moth
world's largest reptile – leatherback turtle
world's smallest insect – fairy fly (a tiny wasp)
world's largest mammal – blue whale
world's largest spider – bird-eating spider
world's smallest reptile – dwarf gecko
world's largest amphibian – giant salamander
world's largest land mammal (weight) – African elephant
world's largest fish – whale shark
world's smallest beetle – hairy winged beetle (trichopteragidae)
world's smallest primate – Philippine tarsier or pygmy mouse lemur
world's largest land mammal (height) – giraffe
world's largest insect (length) – Malayan stick insect (phobaeticus serratipes)
world's smallest bird – bee hummingbird
world's largest rodent - capybara
world's smallest fish – stout infantfish
world's largest butterfly - Queen Alexandra's birdwing
world's largest mollusc – giant squid
world's largest bird – North African ostrich
world's smallest rodent – pygmy gerboa
world's smallest butterfly - Sinai baton blue
world's largest crustacean – Japanese spider crab
world's largest primate – mountain gorilla
world's smallest mammal – bumblebee bat
world's largest beetle – South American longhorn

Source: various; most of these are debatable, I imagine.