Lee, Try plank lengths of 15' or 20'. width around 5" or 6".
I use Obeche. For a working deck, give the wood a light sparayof matte black and rub down until you get a shade of grey that you like. After laying, caulking etc use as many coats of sprayed matte lacquer as you feel appropriate. Obeche absorbs this like a sponge, so be prepared to use up to a dozen coats. BY.