Size and Weight:
About the Elephant Bird (Aepyornis):
First things first: Aepyornis isn't known as the Elephant Bird because it was as big as an elephant, but because (by early, mythical accounts) it was big enough to seize and carry off a baby elephant. At 10 feet tall and close to half a ton, Aepyornis was certainly one of the biggest prehistoric birds of all time; its eggs alone were over 100 times bigger than chicken eggs, by volume, and surely provided a nourishing meal for whichever animal or human being was lucky enough to make off with one.
Like another famous bird, the Dodo, the Elephant Bird persisted well into historical times, only going extinct by the 16th or 17th century. Part of the reason this bird was able to grow to such enormous sizes was that it didn't have much competition on the remote island of Madagascar; since it didn't know enough to feel threatened by early humans, it was easily hunted and harassed by them. However, it's also possible that the Elephant Bird was doomed by climate change, or succumbed to diseases carried by rats (which were themselves introduced by human settlers). By the way, the manipulation of fragments of DNA may enable scientists to resurrect the Elephant Bird, a program known as de-extinction.