Happygirlygirl – there are no halachos about hair length. You won’t find anything in the Shulchan Aruch or Mishna Berurah about it.
That being said, there are two aspects to tznius:
1. The technical halachos of which there are very few, such as covering your knees.
2. Using your common sense to realize that something is not tznius. Most things fall in this category, and something can be assur mitsad tznius even if you won’t find a specific halacha saying that it is assur.
There is also a third aspect, which is the fact that we are supposed to follow our Gedolim, so if they say something, we have to do it. As far as I know, Rav Falk is not considered one of the Gedolei hador, so his sefer should not be quoted as though it’s halacha. If someone wants to follow it, that is beautiful, as long as they realize it is not halacha.
In terms of the Gedolei Hador, I have not seen or heard them quoted as saying anything about hair length (which doesn’t mean they haven’t said anything). I may have heard some things about sheitel lengths, but I’m not sure of the specifics. I think they are more meikel for people living in the US than people in Eretz Yisrael. I think I heard something about 2 inches below the shoulder in EY, and 4 inches below in chutz L’aretz, but I am not sure, and in case, it might have only been a psak for specific people. But that was sheitels, not hair.
There is also a fourth aspect – minhag hamakom.
IF long hair is a problem it is because of 2 and possibly 4. If the Gedolim have said anything about it (which I don’t think they have) then it may be a problem in terms of #3 as well. It is almost definitely not a problem in terms of #1, unless someone here knows something I don’t know and would like to share it.