I thought someone had posted this before:
http://www.theweebsite.com/greatkilt/index.html

Roughly, the "correct" amount of material is 3 times your waist or rump (which ever is larger).

For me, it is 4 yards. One of my belted plaids is smaller and one is bigger, but 4 yards works out about right.

My personal warning is:
Don't use material that is TOO LIGHT (in weight/thinkness)!!!
(It would just look like you used a plaid bed sheet).