I remember hearing from a rabbi and other sources about very long tichel being not tznooyah. What are the sources for this?
Almost certainly just someone's judgment that anything that draws too much attention in any way is immodest in some sense. (The kind of thing you'd find in Rabbi Falk's book, a book sharply critiqued by Rabbi Y.H. Henkin.) The traditional halachic sources only address whether a man can pray in the presence of a woman's uncovered body parts; it's a lot harder to dictate exactly what's called "draws too much attention."
Could an incredibly long train-of-a-hat be considered a "burden" and not "clothing" and would therefore be prohibited to wear outside an eruv on shabbos? Hm ... maybe ...
The concept of Tzniut has two parts to it. One is Halachic based, and one is cultural/behavioral based. Many focus so much on the first one that they forget about the 2nd one, which is the most important one. See Shulchan Aruch (O.C. 75) that clearly defines the laws of parts of the body that should covered during the recital of the Shema as "Bemakom shedarka lechasoto" which can be translated "wherever she usually covers". That's a very broad specification of what must be covered! Most of the topics os tzniut are cultural based. Halacha never discusses if a slit in a skirt if fine or not for example. Poskim will discuss these issues based on what they perceive as being modest or not. So it can vary from place to place. The mentalities are very different between the US and Israel for example. Fav E. Falk prohibits the red color garment for example, but Rav Yaakov Kamenetzky z"l allows it because now even refine women use it. So it really depends on the place where one lives, and the attitude of the person wearing it, like most Halachot of Tzniut.