There are a couple of reasons for why the empire didn't last. One of those reasons is that the empire was too great (big). Because the empire was too huge, it was divided into different parts after Alexander's death. He also didn't have an heir to the throne, so when he died, officials fought for power and to become king, which also contributed to the fall of the empire as well.
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About 470,693 square miles (1.22 million km²)
