I know what you've been thinking: "When is he gunna tell me about the world's biggest banjo?"
Well, unreliable sources claim that an object in Branson, Missouri is the World's Largest Banjo, but I doubt it's a real banjo. To qualify as a true banjo, you need vibrating strings and you need these vibrations to be transmitted to a membrane via a bridge for the purpose of amplification. A website describing Branson's disturbing monstrosity makes me suspect that what they have there is a mere sculpture of a banjo:
"The neck holding five fiber optic strings is 47-feet long. A true replica of a collectible Gibson banjo, the huge fiberglass shell has a sturdy frame of over 3,000 pounds of steel."
Perhaps Gibson's factory in the Opry Mills Mall in Nashville (top) holds the record instead. The search continues ... By the way, see Cecilia Conway's book for an extensive analytical treatment of the features that constitute the essence of a banjo.