In the one of the LotR books of the movie (I think it was either the "Official Movie Guide" or "The Art of the Fellowship of the Ring"), there was a conceptual sketch showing the heights & proportions of all the different character types. The Nazgul were extremely tall, even taller than the Elves, I believe. I'll try to find the book and check the actual heights again, but I'm pretty sure it was around 7 feet. To me they just won't look right at 12". The good thing is that a modified figure can be hidden under the robes, with only the armored arms and legs exposed.
Btw, I have the UC mini Ringwraith sword, and as galactiboy said, it's around 1:5 scale. A shame, really, 'cause it would be perfect otherwise. Still, on a 13" tall figure it might not look too big.