Slow taper for me. Thats not to say fast taper rods dont have thier time and place, and they do have better lifting power, but the thing with slower taper rods, is the ability to short stroke them and grind up fish under heaps of drag. I am speaking from a conventional reel point of view here.
If I was using a spinning reel for jigging (which I wouldnt be
) Id go for fast taper. Having the reel under the rod takes away the ability to really grind the fish up when the rod is fully loaded, especially if there is high rails on the boat, too slow a taper and you may need to lean right over the rail for the reel not to touch the rail, so the extra length can be a reel bonus.
It will be harder on you to jig long period and it will be more sore, but it will also mean you can long stroke more easily.
As for length I wouldnt go more than 6' I dont think, but it depends how long you will be jigging, and how big the fish you intend on catching are. If your fishing for long periods, with heavy jigs and big fish, dont shag around, but a conventional and do it properly