Re: What Size Dock Lines do I Need ?
The issue here is the "well weathered". Dock lines don't last for ever and nylon looses a lot of strength over time. For you boat I would go with 1/2. If it's too big the boat won't generate enough force to cause the line to stretch.
For comparison I use
3/8 - 26' powerboat (hauled out for storms)
5/8 - 38' Beneteau
7/8 - 58' powerboat