He was right about the size of the* ocean*, but wrong about the size of the earth.
Actually Columbus was just arguing the minority view about the size of the Earth. he wasnt a crackpot.
wiki:*As far back as the 3rd century BC, Eratosthenes had correctly computed the circumference of the Earth by using simple geometry and studying the shadows cast by objects at two different locations: Alexandria and Syene (modern-day Aswan).[30] Eratosthenes’s results were confirmed by a comparison of stellar observations at Alexandria and Rhodes, carried out by Posidonius in the 1st century BC. These measurements were widely known among scholars, but confusion about the old-fashioned units of distance in which they were expressed had led, in Columbus’s day, to some debate about the exact size of the Earth. From d’Ailly’s Imago Mundi Columbus learned of Alfraganus’s estimate that a degree of latitude (or a degree of longitude along the equator) spanned 56⅔ miles, but did not realize that this was expressed in the Arabic mile rather than the shorter Roman mile with which he was familiar (1,480 m).[31] He therefore estimated the circumference of the Earth to be about 30,200 km, whereas the correct value is 40,000 km (25,000 mi).
Furthermore, most scholars accepted Ptolemy’s estimate that Eurasia spanned 180° longitude, rather than the actual 130° (to the Chinese mainland) or 150° (to Japan at the latitude of Spain). Columbus, for his part, believed the even higher estimate of Marinus of Tyre, which put the longitudinal span of the Eurasian landmass at 225°, leaving only 135° of water. He also believed that Japan (which he called “Cipangu”, following Marco Polo) was much larger, farther to the east from China (“Cathay”), and closer to the equator than it is, and that there were inhabited islands even farther to the east than Japan, including the mythical Antillia, which he thought might lie not much farther to the west than the Azores. In this, he was influenced by the ideas of Florentine astronomer Toscanelli, who corresponded with Columbus before his death in 1482 and who also defended the feasibility of a westward route to Asia.[32]
Columbus therefore estimated the distance from the Canary Islands to Japan to be about 3,000 Italian miles (3,700 km, or 2,300 statute miles).
*
Columbus’s genius came in knowing about and using the trade winds. Not unknown in his day, but generally kept secret.