If you go the foam route, insulation/extruded polystyrene is your best bet. It comes in 4x8 sheets an in various thicknesses(1/4 inch is the thinnest I've seen in stores), sands well, and once sealed accepts a variety of hard coats and paints. You will have need some sort of spine structure or your blade will snap like a twig, especially at that length. Dowels, threaded metal or fiberglass rods, or as RJ suggested, flat metal bars are available in small enough sizes that you wouldn't add any noticeable amount to the overall thickness.
Sand a channel into one side of your foam blade just wide enough for your dowel/rod/bar/ect. Glue it in place and fill the left over space with your filler material of choice then shape the blade segment.
As long as you choose something with relatively low density, wood can also be quite light even at greater lengths. Using wood allows you to make the sword "full tang" which eliminates the need for a support spine. The Rebellion I made for my Dante costume is about an inch taller than I am, which puts it at about 6'3", made entirely out of wood and comes in at approximately 3lbs.
Pretty much any prop sword tutorial can be adapted to give you the basic process of making this specific sword. You can also take a look at Blind Squirrel's build log from when he made the Rayearth swords for more inspiration: http://blindsquirrelprops.com/magic-...th-fuus-sword/