How do we know the size of earth?
Around BC 240 An Egyptian names Eratosthenes calculated Earths circumference. to a good degree of accuracy.
He found that the in deep well Sun was visible on equinox.About 800 km away a stick was showing 7.2 degree shadow on the same day. The places were syon and Alexandria So if 800 KM w a s 7.2 degree 360 will be around 40,000 kilometers..
Later modern methods wee used to survey earth and found this value to be correct.