Right. In fact, Eratosthenes produced an accurate estimate of the Earth's size in the third century BCE. He was about one percent off. He did it by measuring how long it took the sun to "move" a known distance. Those calculations would have been available. Unfortunately, Columbus used an incorrect calculation made a few centuries later by Ptolemy. Likely his mistake was one many humans make; he assumed that science progresses and the more recent calculation was thus the more accurate.