According to the hadith Jami` at-Tirmidhi Book 24 Hadith 1731 my understanding is that a woman has to wear a long garment which is below the ankle by at least a handspan or forearm. (That's what i understand by 'slacken')
Narrated Ibn 'Umar: That the Messenger of Allah (Peace be upon him) said: "Whoever arrogantly drags his garment, Allah will not look at him on the Day of Judgement." So Umm Salamah said: "What should the women do with their hems?" He said: "Slacken them a handspan." So she said: "Then their feet will be uncovered." He said: "Then slacken them a forearm's length and do not add to that."
While according to the fatwa
If it is short and comes a handspan above the ankles, that is not permissible even if she covers her calves and feet with pants or socks, because that is imitating men who are enjoined to make their garments short, above the ankles.
Here they seem to have set handspan in the other direction i.e. it is allowed according to the fatwa to be above the ankle as long as it is not risen by more than handspan from the ankle.
Question:Are Ankles to be covered for women? if not, till what extent the lower garment can be(above the ankles)?