Had a bit of time so started tinkering in Numista's filters. I've only looked for circulating (both standard & commemorative).
There's some super tiny coins from ancient Greece & India at around 3-5mm in size.
Numista also lists a 2mm coin from early Nepal, though the notes say it was cut from the centre of a larger coin & has no photograph.
After the year 1500, there's some more tiny coins from India & the German states at 8-9mm.
If we jump into modern coins, so milled (machine made rather than hand hammered), there's two coins which are 10mm in diameter.
The lighter of the two is this gold coin from Guatemala (N#56607)
The other is this one from Panama (N#20590).
If you're searching for ones still in, or have been in circulation within the last ~10 years or so, there's a few at 14-15mm which you can see here: https://en.numista.com/catalogue/index.php?r=&ct=coin&im1=&im2=&tb=y&tc=y&cat=y&ru=&ca=3&no=&v=&i=&b=&ie=&u=&a=2010-2100&dg=&m=&f=&t=0-15&w=&mt=milled&g=&se=&d=&c=&wi=&sw=
There's coins from the Netherland Antilles, Mexico, Moldova, Kazakhstan etc. I think the one from Moldova was the lightest of those.
