Smallest in size.. In India, there is a 10 paise with 1cm in diameter. Though its legal tender even today, its difficult to see one such coin in circulation.
Until the Euro changeover, the Dutch nickel 10c was definately a leader in the small coin category for regulary used coins. The diameter of the current European 1c is rather small. Legal tender but not used would be the Panaman 2.5c
When Philips engineers were developing the compact disc, they used the Dutch 10 cent as a mask for the center hole. It was the smallest commonly circulated coin at the time. I don't know what coin has that distinction now.
