As far as I know, biggest ancient ships was a 120-150 metres lenght. It's a Roman Kaligula's ships, Egiptian "Tessarakonteres" and Chinese "treasure ships". "Tessarakonteres" was a 130 metres lenght, with 4000 oarsman, 2850 marines and 400 ratings, officers and deckhands.
Arts of "Tessarakonteres":
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Also, one of tradition Russian plot - town or fortress on huge ship:
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I don't know, maybe, this idea based on ancient giants. And, in Russia from middle XIV century to WW1 builted huge transport wooden ships, for rivers (Volga, etc), named a "belyana" (Russian "беляна"). Big ship for one navigation, not tarred, and, very primitive construction. Biggest "belyana" was a 120 metres lenght, and carryng capacity to 800 000 poods, or 13 100 ton. I haven't data about using of "belyana" at the war, but, I think, it was a very stable platform for montage a guns. Maybe, on "belyana" builted a primitive wooden fortress, and, based on this river battleships, in Russian culture appeated a "town/fortress on huge ship".
(Kaligula's ship at the photo 
)
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