Final answer:
The Louisiana Territory, purchased from France in 1803 for $15 million, doubled the size of the United States, making it about half the size of the pre-existing US territory.
Step-by-step explanation:
The Louisiana Purchase doubled the size of the United States territory when it was made in 1803. The vast Louisiana Territory was purchased from France for $15 million, which was considered a significant bargain at the time. Notably, the acquisition at around half the size of the pre-existing United States significantly enlarged the nation, underscoring the magnitude of the Louisiana Purchase.