1

It is related in the journal Yagdil Torah (New York) XXV, that when the Rebbe was asked about the hair length of married women, he responded: "Not to shave the hair; let it be half a tefach." He demonstrated the length of half a tefach by displaying two of his holy fingers. When further asked whether it must be exactly the thickness of two fingers, he responded: "It does not have to be exact to the millimeter."

Sha'arei Halachah U'Minhag IV, p. 141

2

With regard to your writing about the conduct of previous generations concerning cutting of the hair of married women [whether they need to be entirely cut off]:

In general there are many manners of doing so and many customs among pious Jews themselves. Concerning this matter one may cite the saying of our Sages, "They are both completely righteous." In other words, I am referring to customs whose purpose is the ultimate in Yiras Shomayim. Therefore you should clarify in your circumstance what is best and act accordingly.

Excerpt from Igros Kodesh, Vol. XVI, p. 89

3

A great Halachic authority has written (and the debate is well known) that a women should not entirely cut off the hair of her head, and that doing so is prohibited because of nivul, disfigurement (note Nazir 28a [that a husband may say he does not desire a wife with a shaven head] - but there it speaks only of shaving off the hair,) and in Shulchan Aruch, Yoreh Deah 182:5 [shaving off the hair is similarly prohibited] because "A woman should not adorn herself with male adornment."

However, in Zohar II, 268b and Mikdash Melech on Zohar III, 79a - quoted by the Acharonim as law - it states that even with scissors only a minority cut off all their hair.

Excerpt from a letter of the Rebbe, Sivan 5738