Saw a lot of bulky chainmail links that were too different from just a plain circle so hers a model that will save you maybe 5 minutes of work making your own :)

The external diameter in 14mm and the internal is 10mm with a .5mm slit.

With this model in PLA I haven't had any issues with link separation once multiple are connected but of course if you pull hard the slit does open up wider. Also if you're using this model along with a print-in-place you can use the same color and tell them apart just enough when you're looking close because of the different print orientation. 

The .stl file is a single link. The .3mf is a full print bed for a Centauri Carbon (256x256mm) filled with links. I think its about 225 in a print. 

PS I used these specifically for This Bishops Collar the first time I made it and it worked grand.