The largest circulated coin that I have is a coin from Russia dated 1769. This coin is overweight at 76.9g rather than the usual 50g. Since I owned this coin, I've heard of examples that weigh more than 80g! Bit hard to tell but the edge is at least 5.5mm to 6mm. The largest non circulated coin that I have is from China. This is a 12 ounce coin. Bought this way before the Chinese boom. While it doesn't seem to be that big, wait till you compare it with this To put this into perspective, a Lincoln cent is about the same size as an Australian 5 cent coin.
chrsmat71 - yes the 5 kopek coin did circulate from 1757 - 1810. Still very affordable if you aren't looking for XF+ grade. Some dealers like to call them hockey pucks due to the weight. Most coins should be around the 50 gram mark but with variances, they can be anywhere around 45g to 60g. Very rare over the 70g mark.