most windlass sprockets are engraved with their size. You then need to know the make. as some calibrated chain is good for some makes and not for others.
What sort of windlass? That's possibly a good start. FWIW, most important is the outside measurement of the chain, i.e. the inside dimensions of the cavities of the gypsey. I have a spec. drawing somewhere of the different chain dimensions, but I'm sure some Googlefication wouldn't go amiss in your quest!