Thysania agrippina has the greatest wing span of all lepidoptera at up to 12". A female Ornithoptera alexandrae has the greatest of the butterflies at up to 11". Attacus atlas have the largest total wing surface area.
I've heard of Pseudophinx tetrio larva reaching as much as 10", but 6"-8" would probably be more common. Kind of fits as they are thought to mimic coral snakes for protection from predators.
I've reared both, and Argema mittrei larva get up to about 6", Attacus atlas 5".
in term of surface I don't think that atlas is the biggest; there are other Attacus bigger and female of Coscinocera also have a big surface (a late friend, specialist in saturniid, told me he had a female of Coscinocera with the biggest known surface wing, I don't remember which sp)
s'il n'y pas de solution c'est qu'il n'y a pas de problème ! akuna matata ....