If memory serves, the heaviest insect capable of flight is the Atlas Moth,
Attacus atlas. Which has an 11 inch wingspan, and a total mass of 60 grams in the case of an adult female. The moth with the greatest wingspan, but substantially lower mass (I think it's about half that of an Atlas Moth) is the Great Owlet Moth,
Thysania agrippina, which is known to attain a wingspan of just over 12 inches (though rumours persist that the Austrian entomologist Oberthür had a 14½ inch specimen in his collection, which was since lost).
In the case of the giant stick insect
Acrophylla titan from Queensland, males can fly, whilst females are too large to fly, despite possessing functional wings. Females can reach 30 cm in length. The other large Australian stick insect,
Euycnema goliath, the males are also capable of flight, whilst the females are weighed down by a heavy abdomen.
However, the longest stick insect known to science is
Phobaeticus chani from Borneo. This species can attain a length of over 56 cm, as
this page from the Natural History Museum reveals. This species was new to science in 2008.
