It has been written in many places that a square dance skirt 'must be' 6 yards around the bottom. This much fabric on some of the smaller, shorter dancers would pin them to the floor. There are also some instructions, for multiple tired skirts, that call for 11 widths of the fabric to make the bottom tier. That would be 11 times 44 inches or 484 inches which equals 13.4 yards. The 'right' amount of fabric for each tier is determined by the length of each tier, the waist circumference of the wearer, and most important the appearance that the dancer wants.
For a full skirt each of the tiers must be full enough, around the bottom edge of the tier, to form a full flat circle with the dancer in the center. If the tier is too skimpy at the bottom edge, it will not spin properly. Also it may not fit down over the petticoat. If it is too long, the skirt will be too heavy and will fold into the petticoat.
For a prairie length the skirt does not have to be a full circle. Most are about three quarters of a full circle. This depends on the preference of the wearer and the type of fabric. For a less full prairie skirt do the calculations for a full circle and reduce each tier by a forth.