Now that New Zealand announced their plans to make their coins smaller, the Australian 50-cent piece becomes the largest coin I’ve ever seen. Are there any modern coins in common circulation that are larger?
While not in ‘common’ circulation, the Eisenhower Dollar coin is 1/4" wider.
When you use the term “large,” are you referring to diameter, volume or weight?
Not as big as the Ike Dollar, but the Kennedy half-dollar measures 30.6mm, or about 1.2 inches. Like the Eisenhower coins, however, you can’t say that the half-dollar is in comon circuation. They’re actually pretty rare.
All three are perfectly acceptable criteria, though I am most interested in mass.
No numismatist dopers?
I think the qualifier, “in common circulation,” is gonna keep you from finding one heavier. The high cost of making a heavy coin is preventing most countries from making such things these days, and the trend for 30 years has been to ever-decreasing size.
The Ike dollar referred to above weighs 24.6 grams, and has little numismatic value. IT’s just that people tend to put aside coins that are no longer minted. Value-wise, Ikes should still be circulating.
I’d guess the Aussie 50 cent piece, in its current form, will be made smaller rather soon.
The copper Cartwheel tuppence, minted for a few years but always dated 1797, was effing enormous - 40 mm in diameter and 5 mm thick. You’d know if you had a shilling’s worth of those in your pocket. And they stink, too.
If by “rare” you mean not often seen in circulation, I agree. If you mean rare as in numismatic value, that’s not the case.
OK, so the next logical question becomes “what is the SMALLEST (diameter / volume / weight) coin currently in common circulation.?”. I know that there are arguments as to the smallest historical coinage, with examples of 5 mm diameter coins existing. What is the smallest anybody is currently dealing with on a day-to-day basis?
I sincerely hope so.
It’s impossibly big and heavy, especially given that 50c isn’t much these days. I always seem to end up with a pocket full of the bloody things. I hope we do what New Zealand has done. They can get rid of the 5c while they’re at it.
The extremes in my Big Bag O’ Foreign Small Change are the Polish 1 grosz and Lativan 1 santims (both 15mm diameter) and the Kenyan 5 shillings, 30mm diameter (same size and shape as old British 50 pence, which is why I brought loads home 
). I am not sure of the current status of these though.
Looks from this page like the old 5 shillings are no longer used. The old 10 cents were enormous, even bigger than the 5sh.
And what the heck is the 40 shilling coin all about?
Yeah, I meant that they don’t circulate much 