Most line manufacturers recommend that the line diameter to sheave diameter be not less than 7:1 to minimize friction.  So a 1/2" line for example should be used on a 3.5" or larger sheave.  I use 7/16" for my main and headsail halyards, but have 5/16" for my spinnaker halyard.

I've found that lines smaller than 7/16" are harder on the hands when you are pulling on them, oh wait maybe I'm just getting old...

If you have self-tailing winches you will want to check what size line the self-tailing jaws are designed to handle.  If your halyards are lead aft to the cockpit you may also want to check what size line the sheaves on the cabintop can handle.

Kevin

Alan,

I'm using New England Rope Sta-Set and Sampson XLS.  There are lots of new materials being used in rope manufacture, but for my use the old basic polyester lines are fine.  If I were into racing I might consider some of the new stuff, but I'm cheap.

Kevin