David,

Most sailors use 3/16 or smaller and taper it on both ends if using the snuffer (I think marlow is stock). The lengths of the haylards I used on my I 20 were different when using a snuffer vs. straigth pole but it depends on how you rig it. Most I20 sailors that use the snuffer run the halyard from the head of the spin up through the bail on the mast(some attach a small block on the bail) up through the cheek block on the mast down through the bail to a block on the front beam (some run the haylard through the spreaders and some have a bungee loop in front of the spreaders)then through a cleat or spinlock to a block attached to the rear beam with bungee and back to the front beam (some drill through the front beam and put a thru deck bushing on each side especially if you have a self tacker) into the snuffer bag through the patches in the spin and tie off to the last patch. Have fun