Columbus' major error about the size of the planet was that he thought it was smaller.
Step-by-step explanation:
Columbus created a plan in the early 1480s which involved a sailing expedition west towards China. However, he needed funding from the Portugese Kingdom for such an expedition. There was lot of debate about the size of the Earth between Columbus and the Portugese.
According to him, the Earth's circumference was on an average, 18,000 miles.
However, the real circumference was 24,901 miles.
Columbus's major misconception was that it was a very small distance between the two ends (East and West) of the Eurasian landmass. By assuming this, he also ended up shrinking the size of the Earth in his calculations.