I have a 1976 Proof that's still sealed in the OGP, and it measures about 10mm in diameter (best I can do through the plastic.) I'm sure I have one of the 1904 versions somewhere...looking for it to compare. EDIT: found it. They are the same size (or extremely close to being the same size.)
I became fascinated with the "Panama Pill" after reading this thread and decided I had to have one. I just won this one tonight on ebay: https://www.ebay.com/itm/1904-NGC-M...ANAMA-PILL-034-2-1-2-CENTESIMOS-/232569567016