I measured a couple of nails. I can't tell you the size, but one would either be a #5 or a #6, used on light horses, ie: saddle horses, and they are 2 1/2". The only other size I have is heavier shanked larger nail that would be used on a work horse, and I have no idea of the size, but it's 2 3/4" long. I've been retired for a long time and the size on the box the ink has faded off. I would expect that there are nails in the three inch range, but I also suspect the ring isn't made from a horse shoe nail, which in Colonial times would have been hand forged and not a uniform stamped out size, but made by the black smith just for that foot.