Tim,
It's not difficult to figure out the length you need. Double the height of the mast. Add any length for the halyard going back to the cockpit, if it does. Add a bit more for some turns around a winch when the sail's all the way down, plus some tailing length. Add in five or ten feet of "slop factor," just to be sure.
Not really rocket surgery
Unless you know how to properly cut and dress the ends of lines so as to prevent fraying, try not to go
too far over, or you'll have a mess of extra halyard laying-about all the time--particularly when the mainsail is up. If you do know how to dress line ends, you can always trim it to length, yourself. (It's not hard, btw.)
Btw: It's a "main halyard." The "mainsheet" is the line that goes between the boom and, well, somewhere below the boom, to control mainsail angle and leach tension. That "pulley" at the top of the mast is more properly called a masthead sheave (pronounced "shiv"). (In this case: The "mainsail masthead sheave." There'll also be one-or-more jib and spinnaker masthead sheaves.) Finally: A 3/8" halyard on a 19 ft. boat? Are you certain of that? Our 30' boat had only a 3/8" halyard before I replaced it with higher tech 8mm stuff.
Jim
