You're quite right, Angie, it had been known since the time of the ancient Greeks that the Earth was a spheroid. And within the limits of the methods that were available, there was a calculation of the size of the Earth that was pretty good.
Columbus however didn't know how large the Earth was, and refused to accept the best calculations of its size. It's why the learned folk didn't want to support his voyage - not because they thought he'd drop off the edge, but that he would run out of food and water before he got to Asia. In fact, that's what would have happened if the Americas hadn't been there. He maintained to his dying day that he *had* reached islands off the coast of Asia.