Re: Optimal halyard length?
I am a huge fan of dyneema lines for pretty much everything, but in this case I don't really see the point. To get a large enough dyneema line to be able to handle you have to go up in size far past the needed strength, even taking into account stretch. Quite frankly 7/64th amsteel would be more than strong enough, but it would be to small to grab.
If you feel like learning to splice, you could go with a 7/64th dyneema halyard spliced into a sta-set tail. But I think this is a lot of work for minimal gain on a non-race boat. Though since the dyneema is so cheap it would save a little money.