What is the largest taildragger in history?
If you are going by MTOW, I believe that would be the Junkers Ju 390, which had a maximum takeoff weight of 75,500 kg.



Junkers Ju 390, By Bundesarchiv, Bild 141-0072 / CC-BY-SA 3.0, CC BY-SA 3.0 de, Link

Sadly, only a couple of these were built. If you are going by the wingspan, that would be the Tupolev ANT-20 'Maksim Gorki', which had a wingspan of 63m



ANT-20, By Unknown - http://www.dkvnukovo.ru/photos/museum/photos/35.jpg, Public Domain, Link