I agree with the 5/16HT and 5/16 3-strand nylon recommendations IF you do not have or plan to own a
windlass. If you think a
windlass will be in your future...pretend you were selecting one now and make sure the chain is compatible with the gypsy on the windlass you would select but at least 5/16.
I agree with the 35' for the Rocna but note that
Fortress only recommends very short chain lengths as does
Danforth. I would get 20 ft. and keep it easy to handle. 200ft. of nylon should be plenty for the Bay.