Todd,

If you taper both sides, there's a good chance that you'll only be able to taper a very small length on the retreival end. A lot of that retreival end is pulled through into the bag and out the snuffer gromett and you don't want the crew yanking on 12-strand.

KrashKrantz has the dimensions of my halyard which is perfect. Trey could also chime in here. As for material, I recommend the 12strand amsteel spliced into 4mm robline racing sheet. Its the best halyard I've ever had. Get some Holmenkol and soak the 12strand section in a bucket overnight.