Which US purchase of 1803 doubled the nation's size?

The Louisiana Purchase - about 828,000 square miles from France for $15 million, which works out at roughly three cents an acre and is routinely called the best land deal in history.

Nobody meant it to happen. The American delegation went to Paris to buy New Orleans, because the port controlled the mouth of the Mississippi and therefore the entire western economy, and were offered the whole territory instead. Napoleon needed money for a European war and had just lost an army in Haiti, where a slave revolt had defeated three of them; without Haiti as a base the North American plan was pointless. So the largest land transfer in American history is a direct consequence of a successful slave rebellion.

Jefferson believed the Constitution gave him no power to do it. He considered an amendment, decided there was no time, and went ahead - a president with a strict-construction philosophy making the most expansive executive decision to that date. What was actually bought was France's claim, not the land: it was occupied by dozens of nations who had not been consulted, and acquiring it in practice took a century of treaties, wars and removals.