Q: What is the 'awrah of a female in prayer?
A: The 'awrah is considered that which must be covered of the body.
The minimum that must be covered for girls ages 7 to puberty for prayer to be valid, is everything between the navel and the knees.
Women, the age of puberty and older, must be ENTIRELY covered apart from their face.
It is preferred that they pray in three garments, a dress, a khimar, and a jilbab.
Outside of prayer, the face is considered a part of a woman's 'awrah. Her voice, however, is not.
Source: Sharh al-Muntaha, Kashf al-Mukhadarat