from a Theravada bhikkhuni…
not sure how much help I can be because our robe regime consists of five pieces and has other slight variations. But here goes:
Are all uttarāsaṅgas and saṅghāṭis the same size (6x9, regardless of how one measures a sugatavidatthi) for all monastics? Is it allowed to trim them or would that constitute ruining a gift given on faith and waisting robe-cloth?
No, there is a variation in size, the correct size can be determined by holding the top of your robe above your head and measuring to the feet. The 6 x 9 measurement isn’t used in practice in the Theravada world because of a bad interpretation of the commentary, which uses carpentry measurements from the commentary to the kutikara sanghadisesa rule to measure cloth, leading to a huge measurement. Also noting 6 x 9 is the Max size, I.e. for giants.
Yes, you can trim them (I know short nuns who do), but if you trim too much it will wreck the paddyfield pattern & when you are in a group of people who all look the same, minor differences can stand out like a sore thumb. Personally I think making a shorter one from scratch would be the more aesthetic option.
It’s not wasting robe cloth if it works, but “think twice, cut once” holds good? I would rather never cut anything that is already made up, but that is practical and not vinaya. Also, they should still be worn in such a manner that they finish at least 4 fingerbreadths below the knee (according to commentary).
It’s not in the vinaya, this is my own calculation, but in practical terms around 3 x 6 [hattha] (120cm x 240cm) max is reasonable. Compare 2.5 x 6 [hattha] max for the bathing cloth in vinaya.
Cmy says it should finish 8 fingerbreadths below the knee; the basic point is that it covers your knees fully while sitting and standing etc. Normally the Theravada one is mid-shin.
I wear my sankacchika and anataravasaka. I tend to use my sanghati as a shawl because it is warm & in summer I will pile on both uttarasanga and sanghati as sheets.
Nuns will often determine their jacket as sankacchika (where five robes not three), or alternatively, there is a Thai-style one-shoulder under jacket thing that nuns sometimes determine as sankacchika. Personally, I determine my actual sankacchika (a flat piece of cloth) as my sankacchika, but I am in the absolute minority.
For monks, these are not to be worn according to the khandakas, as they are “kancuka”, which is a type of lay clothing. But keeping them as extra is basically universal in cold-climate areas.
I stayed in a place where it snowed; we wore cotton and just layered stuff underneath. Decent fabrics are actually really hard to find in Australia, I doubt I would have gotten permission to spend a few hundred dollars on a proper woolen one. The Tibetans just wear wool, they normally buy them in India?
These days my robes are made out of heavier repurposed fabric including flannelette, which as an EXCELLENT DECISION for life outside of the tropics.
In addition to what Bhante Khemarato has stated, in Thailand, there is a bit of a difference with how these are dyed as well, as sometimes they are redder because they were dyed with clay not jackfruit? (no firsthand experience). But there is no difference with synthetic dyes.
Nuns have a slightly different bathing cloth regime, as ours is the cloth which is worn when bathing in public (also used by laypeople in South and Southeast Asia) & is a permanent requirement. I don’t normally bathe in public so the main reason I use mine is because I’m washing my lower robe.
I think you kind of need to ask a real person, ideally someone who is experienced and has stayed in many places. Youtube has video (search Buddhist robe), but until you see it in real life, it is not possible to truly understand, as it is about “technique” as much as anything & minor variations in where the roll sits can change things.
I know 5 ish main styles (Thai forest, Burmese, Thai “hom gone” (?spelling), Sri Lankan, Chinese). Even within the Thai forest tradition, there are still about 3 minor variants on the lower robe, for example. Also, even to fold the sanghati is an art that is not in books.