Is there a requirement on how long tzitzis must be? I was wondering because there seems to be lots of people who wear rather long tzitziyos.
As in the link that @Double AA posted in the comment the Shulchan Oruch writes:
ולמעלה אין להם שיעור
Meaning there's no maximum length. However, the RamBaM writes הלכות ציצית פרק א ט that:
יתכוון להיות הכרוך שליש, והענף שני שלישים
That the wound section should be 1/3 and the loose section 2/3, but it does not invalidate the mitzvah if this is not the case.
So there is a recommended length which will depend on the thickness of the threads and the density of the windings, but no absolute maximum length.
(As your question seems to relate to maximum length, I didn't mention minimum)
Shulchan Aruch Orach Chayim, 11, 4:
ד אֹרֶךְ הַחוּטִים הַשְּׁמֹנָה אֵין פָּחוֹת מֵאַרְבַּע גּוּדָלִים, וְיֵשׁ אוֹמְרִים י''ב גּוּדָלִים, וְכֵן נוֹהֲגִין. ... וְשִׁעוּר הַנִּזְכָּר יִהְיֶה בַּצִּיצִית לְאַחַר שֶׁנִּקְשַׁר מִלְּבַד מַה שֶּׁמּוּנָח עַל קֶרֶן הַבֶּגֶד (סְבָרַת בֵּית יוֹסֵף).
The length of the threads must be no less than four inches. The measurement of the inch is the width of the inch at the interphalangeal joint. One opinion says twelve inches and this is our custom. this size is the size of the tzitzit after the tying, and concern the part which is beyond the edge of the garment.
(Rambam wrote Hilchot Tzitzit 1, 6 that the length of the eight threads is no less than four fingers. In Gemara equivalences are stated 4 great finger = 5 medium = 6 little)
We learn this size requirement from Gemara. This Halacha is discussed between Beth Shammai and Beth Hillel and that Rav Papa ruled as Beth Shammai in Menachot 41a, four (the unity is not written). Rambam understood that this size is from the garment edge up to the extremity (as the first opinion of Shulchan Aruch), and Rabenu Tam understood that this size is from the edge up to the free fringe (as the second opinion cited in Shulchan Aruch).
There is no maximal size requirement (Rambam 1, 6, SA 11, 4), Mishna Berura sk 21 wrote that the custom is to make the Tzitzit longer than the minimal size, so if a thread will be teared, the minimal required size remains. If someone want to cut the threads (obviously, not short of the minimal size), this is allowed.
I have hear from a Rav, but I don't remember the source, that with very long Tshitsit, may be "bizuy mitsav" because they touch the ground and will be soiled and people will tread on them.