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An 8 yarder would only use 7 yards for you. That would give you the optimal swing, but I think a 5 yard kilt would still look nice on just about anyone. Not quite the same swish, but still very nice. the pleating would be a little creative though so if you're looking for pleating to strip or sett you'll want the 7 yards.

Rather than think about how it will swing, think about how you will use it. If you intend to dress up for special events the 7 yard kilt would look better. If you just want something to wear around town then the 5 yarder will do fine.