Capt Matt
guessing on 5mm or 7mm i also bought some chain from Michales
While called "'chain plates' they are not actually chain.
The lower deadeye is held in a loop that's pinched into a lopsdied figure of eight shape. The upper half the diameter of the deadeye groove. The narrow "neck" of the figure eight goes through the cannel.
An iron loop passes through the bottom eye. This loop is often a sort of egg-shape, if distended. Another loop is made up through the first and its end is bolted to the hull (or to a third link bolted to the hull).
Since these are loops and not a single bit of bar stock, they are referred to as "chain links."
Assembled:
Pieces-parts
Actual linked chain did see some use, but only after such chain became commercially available Shipbuilders tended to stay with the old pattern, though, as it was known and proven.
From memory, Constitution's lower eyes are 16" diameter, whic his going to be a hair larger than 1/8", about 4mm to scale. Some of the eurokits use 7mm so that they can use upper 5mm deadeyes.
YMMV