I've made cuff links - bar/chain. Mine were just under a half inch
(chain) and the bar was about the same size. Since the bar goes
horizontally across the button hole, it doesn't need to be very lon,
but then, it would depend on the size of the cuff link itself.
I have been asked to make a pair of bar and chain cufflinks (old
school). After much searching I'm not able to find any reference to
how long the chain or how long the bar should be. I did find one book
that said the chain should be 5 links long, but they neglected to
state the diameter of the links. It seems as though there should be a
standard measurement for such things.