A summary of guidelines for modest skirts at ModestAnytime.com according to halacha.
Skirts should be loose and totally relaxed around the hips and thighs so that the complete area down to the end of the knees is hidden and totally camouflaged by the skirt. If the skirt is fitted or even semi-fitted, it is not Kosher, as this displays the general shape of the area. A kosher skirt should have at least 4" of ease at the widest part of the hip. A kosher skirt should also be at least 4" below the knee and 6" to be mehudar (refined). A kosher skirt should not extend within 4 inches above the ankle. At ModestAnytime.com, we try to adhere to these standards. Our skirt styles are all A-line and flair. We offer both skirt lengths (4" or 6" below the knee) to our clients.
Reference: Modesty, an Adornment for Life, and The Tznius Handbook by Rabbi Eliyahu Falk.