Question: Who was the individual who is credited with establishing the size of earth?
Disambiguated questions and answers: question: Who was the individual who is credited with establishing the size of earth around 240 BC?
Who was the individual who is credited with establishing the size of earth  during the middle ages?
Who was the individual who is credited with establishing the size of earth using the star Canopus?
Who was the individual who is credited with establishing the size of earth by adding to Eratosthene's estimate?
answer: Eratosthenes of Cyrene
Eratosthenes
al-Biruni
Abū Rayḥān Muḥammad ibn Aḥmad Al-Bīrūnī
Biruni
Posidonius
Hipparchus
Hipparchus of Nicaea
Natural Questions answer: Eratosthenes (from the Wikipedia article "Heliocentrism")