5/16 is plenty strong enough, but 3/8 inch will feel better on your hand for the sheets. For the halyards match what you have or measure the shiv i the mast. Too big and it the halyard will bind. Make sure you don't cheap out on the halyard especially or they will stretch. Spens money on sailing glo...

In heavier winds a boom brake or preventer is a bad idea. Many people used to use these in the 70s and 80s to prevent the boom from flying over unannounced. The issue is IF the wind gets behind the sale and you have a preventer on, the boom will flop over as it can, cause the boat to round up and pi...