What other country does the most populated nation in the world share a border with and how long is the border between the two nations?
Nation and country are synonyms
SELECT T2.Country2, T2.Length FROM country AS T1 INNER JOIN borders AS T2 ON T1.Code = T2.Country1 INNER JOIN country AS T3 ON T3.Code = T2.Country2 WHERE T1.Name = ( SELECT Name FROM country ORDER BY Population DESC LIMIT 1 )