Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says that the Messenger of Allah (peace and blessings be upon him) said: "Shaving the head is not required of women; rather, what is required of women is cutting the hair short."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9404
Ibn Umar (may Allah be pleased with them both) says, concerning a woman in the state of ihram, that she should cut an amount of her hair equal to the index finger. And it is reported from Sayyidah Aishah (may Allah be pleased with her): We used to perform Hajj and Umrah, and we would not cut more than a finger's length.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9407