For spinnaker sheets, they need to be at least twice as long as the boat - I think I may use 48 - 50 feet, but am not sure. When you are rigging the sheets and halyard to the spinnaker is the time you need the maximum length for each sheet. the guy has to be able to extend around the bow and back into the cockpit while still having enough sheet on the windward side for the spinnaker flyer to hold on to.
As to the diameter, this depends on your wind conditions. My thickest sheets are 5/16 - easier to hold on to and strong enough in plenty of wind. For lighter days, I use 1/4 inch, however, this does not have the strength and is hard to grip in stronger wind. Some boats use tapered sheets - lighter at the sail end and easier to grip in the hands of the spinnaker flyer.
According to the 1980 parts catalog (library/historical docs), the spec was 5/16 for both 60’ and 34’), and 5/16 seems to be the standard for a LOT of lines on this boat.
That said, parts catalog sats 3/8” for gennies at 25’
link to catalog - hope this works. Page 12
I hope someone could jump in with a quick response before I head to Defender. I'm replacing my sheets and It looks like my old lines are 10mm. Should I stick with 10mm for both the main and jib sheets? I don't race so I don't need anything fancy. I was looking on the neropes site and they are showing for that size of a boat should be 10 mm for the main and 8 mm for the jib. Any thoughts? Thanks in advanced.