An all graphite rod will begin to have a memory if you use it on a DR, and could fray after using it locked down on a rigger after a while. You could go with a graphite/composite blend that will offer good action and strength that wont develop a memory.

As far as length, I like to use a 10'6" rod because with that length it loads up really nice while on the downrigger, gives you extra play and leverage on the fish, and helps you clear a longer setup out of the water when landing a fish.

The rod shoudl also be built with quality guides and cork that will withstand the rigors of downrigger fishing and general abuse of saltwater fishing.