The Acteaeom Beetle, more commonly known the Rhinoceros Beetle, is the heaviest insect in the world. As for length specifically, Arthropleura, the earliest known ancestor of centipedes and millipedes is the largest, but it evolved at least 400 million years ago, and died out some 235 million years ago. It grew to eight feet long, but despite them being monstrous in size, they'd be harmless to us, as they ate wood off logs, and tree bark.