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.
Nick Lubar
Fleet 29