My answer to this question:
How far can a ship "see" in 5e?
prompted the question whether the Forgotten Realms are on a disc or a planet. It is a planet, as evident from the answer to this question:
Where does the Sun go at night?
However, I’m wondering what size this planet is. This information would be relevant to calculate sight ranges. Barring any contradictory information I would assume, similar to the size of earth, but my question is, if there is any official information to corroborate this or give a different size. Information for 5E is preferable, since the other question was also specifically about 5E, although I suppose that this probably has not changed between editions, so prior editions are valid if there is nothing specifically from 5E.