With a 12-inch wingspan, the white witch moth (Thysania agrappina) is the largest in the world. It's freakin' huge! In contrast, the roughly 300 species in the Nepticulid family have a wingspan between about 1/8 to 2/5 of an inch.
The Chinese giant salamander (Andrias davidianus) can grow 3.3 feet long (or 5 feet long according to this source). Salamanders of the Thorius genus, however, can be fully grown at less than an inch long.
Rosemary Mosco is a field naturalist and the artist behind the webcomic Bird and Moon. She made this chart illustrating the startling size differences between several related animals.