I think the chain was 6mm. To tension, I use a block and tackle arrangement wherby the chain can be slackened off from the foredeck if necessary (having first taken the strain off the forestay). I have very rarely needed to do this, but it allows me to retract the bowsprit if required.
To prevent the chain snagging on mooring lines when using a swinging mooring in tidal conditions, I use a primary line attached directly to the bow eye (almost at water level).
I know you shouldn't fit a stainless shackle with a galvanised chain, but in my Winkle Brig situation where the boat is kept on the trailer between outings it hasn't been a problem for me as the chain is not subjected to continuous salt water contact so galvanic action is minimal (and can be checked annually). I find galvanised shackles to be much more problematical.