I'd say any length between 4 and 5 yards of double width fabric will be fine. I have a 38" waist and consider my 5 yard belted plaid to be a bit on the long side for my personal taste. It works fine but I'm thinking of lopping off a yard of fabric so I don't have as much on top to deal with.

BTW, as far as the belted plaid (great kilt) goes, there's nothing to "make". It's just a length of wool fabric that's strategically pleated and wrapped.