Columbus knew the distance around the globe. That was accurately determined by ancient Greeks. What Columbus (and the Greeks) did not know was the size of land

Columbus knew the distance around the globe. That was accurately determined by ancient Greeks. What Columbus (and the Greeks) did not know was the size of landmasses in proportion to each other. They thought Europe was much, much larger than it was. Large enough to leave little room for ocean between the Azores and East Asia.