My greatkilt is about 5-51/2 yards by 60". I'm a well padded (270lbs) 6'. That length seems to work fine for me. Truth be told I could get by with less. I've heard of the drawstring & belt loop idea, and it makes sense and seems to work well. I have a "cheater" great kilt that has the pleats sewn to a cloth belt. It's not strictly correct, but it's a lot easier to put on in the morning in the tent. For demonstration purposes, or when I'm feeling energetic, I have a length of tartan that I use as a great kilt. A few years back, someone showed me a method of "throwing on" the great kilt. You pleat the end of the plaid up into one hand, drape it over your left shoulder, anchoring it with your chin. Then you buckle your belt, a bit loosely, around you. Following that you arrange your aprons into position, and fasten the plaid as you wish. A bit cumbersome, but it works.
My favorite description of the great kilt is that it's a way to wear 5 or more yards of wool around your waist and not look like a bag of rags, from the front.