In one of our recent D&D sessions, it actually became relevant exactly what size were the coins the characters were carrying. Thanks to the internet, and especially to the miraculous Wolfram Alpha, I can now give a categorical answer to that age-old question.
The assumed weight of coin is 50 to the pound, which means that any coin, regardless of its metal, weighs 9.07 grams. With that information, and knowing the density of the three primary metals for coinage (gold, silver, and copper), Wolfram Alpha gives us the respective diameters for cylinders 2mm high (or thick, if you prefer) of 17.3279mm, 23.421mm, and 25.419mm respectively.
So, there you are. That's how big your coins are.