There are no hard and fast halachot regarding foot covering. According to most halachic decisors, the knee should be covered, even in a seated position. Covering the rest of the leg is more a matter of debate. As such, foot and leg covering follow the standard practice of the religious community with which one affiliates.
For more details on modesty of dress, we recommend the book Understanding Tzniut by Rav Yehuda Henkin, our rabbinic supervisor. The book incorporates his article “Contemporary Tseni’ut,” which was printed in Tradition magazine (vol. 37 no. 3, Fall 2003) and is available online here. Deracheha is another excellent online resource.