The largest coin ever minted by the US Mint was a gold Half Union pattern in 1877, weighing 83.45 grams, and 51.1 mm in diameter. The largest coin actually issued by the mint was the Panama-Pacific Exposition $50 gold commemorative, at 83.572 grams and 44 mm. An octagonal version of the coin was slightly larger, measuring 45 mm at its widest point. ^ The United States Mint: Frequently Asked Questions.