My Laird's plaids are about 5 yards long. A smaller man could get away with less. I would say 4 yards would be plenty for you.
I did a double-take when I saw that, but it might make sense. Using 4 yards and folded in half length-wise, it would be 6 feet long, which would be about 3 feet of drape in front and behind (when worn over the shoulder). I guess I'll just have to play with it and see how that looks if I unfold the length and wrap it around my chest. 12 feet seems awfully long for that, but maybe it's not.

Are those images for a full (belted) plaid, or a day plaid? Using 4 yards of double-width cloth seems like it would be enough for a belted plaid, and I was under the impression that a day plaid was much smaller... like the size of a blanket, as Chas pointed out.