This is a pack of 10 x copper rivets and washers in '6 gauge' size.
They measure 21mm in total, with the stem being 19mm long. The base & washer measure 13mm across. The width of the shaft is 4.4mm.
Copper rivets are fitted by pushing the stem through a hole in your leather, adding the washer and tapping it down flush with your leather. Using end cutters you then cut off any excess stem length, leaving yourself with approx 3-4mm to hammer down into a dome shape using a 6 guage copper rivet fixing tool.