how long should I make my circle skirt?

Circle skirt need a bit of length to help keep them down and to give them the correct amount of flow and drape. Three inches above the knee would look too much like you're off to a figure skating competition and worse, an errant breeze might expose more than you want. The longer a circle skirt is the more classy and elegant it looks. Don't get hung up on some "ideal" or "golden" measurement -some skirts look better longer and other look better shorter. Base the length of each skirt on the silhouette and how the proportions of the silhouette work with your body. Too much volume in too short a skirt will through off the proportions of your over-all silhouette -such as a too short full skirt.