Red CoatThat’s a beauty! Well done. And you raise an interesting question.
It’s complicated a bit by the existence of many modern ‘collectible’ and ‘commemorative’ coins that theoretically could circulate as legal tender but were never intended to do so (including some obscure countries in whose name coins have been struck by commercial mints). Also, there are numerous large and oddly-shaped metal items that have circulated as currency in various places which might arguably be called coins, despite not being the disc-shaped things we generally regard as such.
I don’t know for sure, but I think the leading contender for what we would recognise as a coin that definitely circulated (albeit as a bullion coin) would be the German States Principality of Brunswick-Wolfenbüttel 16 Thaler struck by Prince Julius in 1588.
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The coin was struck in silver with a diameter of 72mm (2.8 inches), a thickness of 11mm (0.43 inches) and it weighed a whopping great 462g (just over 1 pound).