Final answer:
The Louisiana Purchase had the greatest impact on the size of the United States, as it doubled the nation's size and started its major westward expansion.
Step-by-step explanation:
The event that had the greatest impact on the size of the United States was B. The Louisiana Purchase. In 1803, under the presidency of Thomas Jefferson, the United States bought the Louisiana territory from France for $15 million. This acquisition effectively doubled the size of the nation, starting a substantial expansion westward and was viewed as one of the greatest real estate deals in American history. It laid the groundwork for further exploration, economic exploitation, and territorial acquisitions that shaped the young nation's destiny and identity.
Although the Gadsden Purchase, the Oregon Treaty, and the annexation of Texas were significant in their own right, none contributed to as dramatic an increase in land mass as the Louisiana Purchase. This purchase greatly enhanced the vision of an agrarian republic and provided for the expansion of American influence and power across the continent.