The best I have come up with the size of the bottom ring is 10mm OD, and 1mm thickness. I maybe off a bit since I was measuring a ring that was already used and therefore enlarged a bit.
FWIW: me, myself, and I wouldn't order a 10mm o-ring for there..not yet anyway
Time will tell..Like I said above, my mini are running the second one from the smallest..(this is just a test)