Hi Gene,

Especially with today's high tech rope, 5/16 would be fine, I would think... I have 3/8 Warpspeed, and I feel it's more than enough, I like oversized on the main halyard primarily for going up the mast myself... You might check the West Advisor or Annapolis Performance Sailing's website, I believe they have some sort of recommended diameter selection guide...

best regards,

Jon