While performing prayers, a woman must cover her entire body, even her head and hair; and based on obligatory precaution, she must cover her body in a way that even she cannot see it. Therefore, if she wears a chador in a way that she can see her body, it is problematic [i.e. based on obligatory precaution, a woman must not wear a chador in such a way]. However, it is not necessary for a woman to cover her face, her hands below the wrists, or her feet below the ankles.
To be certain that she has covered the obligatory areas, she must also cover a little of the sides of her face and a little of the area below her wrists and ankles.