The typical fly plaid you get from most commercial suppliers is about 40" square, but I've custom made them for the taller gent up to 54" square. I think this is a good range. 40" would be the minimum, and 54" is really as large as you can get using double-width cloth without having to seam the fabric together.

I was talking with a gentleman the other day who wondered if it would be improper to wear a flay plaid of one tartan with a kilt of another. I told him while it might not be "improper" it likely simply wouldn't look good. Something doesn't necessarily have to break the "rules of tradition" to break the rules of fashion!

In the case of a fly plaid, I think if you attempted to use a long, narrow length of tartan, akin to a lady's sash, in the same manner, you'd end up looking like you were attempting to imitate a "Scottish costume" rather than actually wearing the "real deal." That being said, I think Alan's length of 2.5 yards of 30" wide cloth might make a decent "shoulder plaid."

What I call the shoulder plaid is less formal than the fly plaid, and really has no set deminsions, though it should be fairly large. It's basically a length of heavy tartan cloth that is folded up into something managable and tossed over one shoulder. Think of the country gentleman going out to walk on his estate and taking a plaid with him to perhaps spread out for a picnic, or to wrap up in if it gets cold later on. You don't see this style of plaid very often any more, but it is traditional, and much more suited to day wear.

What I would do would be to just put a simple hem on the long ends (assuming they are not a selvedge edge) and then fringe the two short ends, and that is it.

Aye,
Matt