This is a pack of 10 x solid brass rivets and washers in '8 gauge' size.
They measure 19mm long in total, with the stem being 17mm long. The base and washer measure 11.5mm across.
The diameter of the stem is 3.9mm, so it's best to punch a 4mm hole in your leather.
These rivets are fitted by pushing the stem through a hole in your leather, adding the washer and tapping it down flush with your leather (this will be a tight fit).
Using bolt cutters, you then cut off any excess stem length, leaving yourself with approx 3mm to hammer down into a dome shape using a copper rivet fixing tool.
Please note that solid brass is slightly harder than pure copper, so you may want to consider the copper version if you haven't used this type of rivet before.