How big are a butterfly's wings in relation to its body?

It depends on what species of butterfly it is. The smallest is the Western Pygmy blue from Africa, which has a half an inch wingspan. The Largest butterfly is the Queen Alexandria's Birdwing, it has a wingspan of 11 inches and is endangered and it lives in New Guinea.

More information at http://www.entomon.net/largest-smallest-butterfly-moth.htm