Nirn (Earth-4552)

Nirn (Q: "Realm", pronounced [ˈnirn]) was the Quenya name for the world as a whole. Nirn was created within the Eä by Eru Ilúvatar, and then shaped by the Valar into a single landmass known as Mähruar, but with the ages, it separated into various continents such as Middle-Earth, Tamriel, Essos, Westeros, Atmora, Aman, Pyandonea and Sothoryos.

Theories
Carecalmo theorized that Nirn's diameter was around 886,700,000,000 miles.