well, i am no tribal costuming expert but logic tells me the length of the scarf depends on the proportions of the rest of the costume and what you need to balance our your body shape/type.
it is hard to say without seeing the costume. do you have pics? do you mean like this
or
if you want the veil/scarf very long, you might want to tuck the bottom corners into the sides of your belt to place emphasis on your hip movements when viewed from the back.
i prefer natural fibers when ever possible and find lurex itchy. again, i would have to see the rest of your costume to give you an opinion of what "goes".
sorry if this is not very helpful. i need some sleep!