No one person "determined" the size of the Earth correctly. It was a process of scientific discovery that involved many contributions over centuries. Here's a breakdown of key figures and events:

* Ancient Greeks: Eratosthenes (circa 240 BC) made the first remarkably accurate measurement of Earth's circumference. His method involved comparing the angle of the sun's rays at two different locations and knowing the distance between them.

* Medieval Arab Scholars: During the Middle Ages, Muslim scholars refined the measurement of Earth's circumference and improved astronomical calculations.

* Age of Exploration: With the voyages of explorers like Christopher Columbus and Ferdinand Magellan, more accurate measurements were made based on astronomical observations and triangulation techniques.

* Modern Era: With the development of accurate surveying tools and satellites, the size of the Earth has been determined with incredible precision.

Therefore, the size of the Earth is determined by the collective efforts of many scientists, mathematicians, and explorers throughout history.