May we ask why you are asking? If you are in a monastic education program, these topics will possibly be touched on. If you aren’t in such a program and want to learn about these things, the first step would be to look for one. As it was me who first really pointed out the existence of the ~45cm sugata vidatthi to the English speaking crowd, I assume it is my earlier post(s) to which you are referring on this point.
I can’t recommend the BMC in general as an Indological text, 25cm is the Thai kheup measurement from the 1913 Vinayamukha, and not an Indian measure. The ~45cm is the Nanshan commentarial measure (based on the earlier tradition), as I have previously pointed out. AFAIK the sugata vidatthi was pinned to Chinese measurements as early as the 7th century CE, which makes it pretty easy to trace as Chinese kept extensive almanacs and records of weights and measures. This measure is actively in use in the vinaya schools in East Asia, opposed to the Thai kheup measure, which has always been mostly speculative, combined with the fact that this measure appears to be unknown to the Pali commentarial tradition itself.
I was once flashed by a monk who took the Thai kheup seriously and ended up exposing himself because his robe wasn’t big enough- hence the beginning of my concern about this issue.
To really have a good handle on the measurements, it’s good if the “expert” could please study the works of Daoxuan 道宣 (596–667) and Yijing 義淨 (635–713) first. If you want a second opinion, try these people (contact form for Yidesi temple) or Nanlin people, (address retracted after moderator request).
I just use the pillow from the shop?
The most meticulous way with the robe is to have at least five fully cut sections. You probably don’t want four because it needs to be an odd number to have a back panel. So it would need to be 5,7, or 9 sections. See the study linked:
6007Venerable Paññā Nanda (1).pdf (2.6 MB)
Not everywhere is meticulous about it (my guess is that the vast majority of places don’t really care due to a proliferation of commercially manufactured robes). The robes from the shops often have “faked” cut sections, meaning that there are actually only three or so cut pieces, but the overall visual effect is always a robe of at least five panels.
To get a good handle on robe related issues, I would recommend a stint as a monastic store officer. It forces you to think through robe sizes logically. I also briefly taught robe sewing to nuns. That’s the only reason I know so much about these things. If we think about the original reasons why there were restrictions on length and height, it’s good to remember that people in the Buddha’s time may have equated many pleats and a train on their clothing with status or sensuality: unless you actually have this type of abnormal mentality of the priestly or ruling classes that you want to set up a dancer’s outfit or a trailing cloak for yourself, everything that comes from the shops is pretty much ok in terms of size if used for its intended purpose (in fact, East Asian sitting cloths are often slightly larger than the skimpy Theravada versions).
There probably isn’t any harm in the laity choosing to wear clothing of moderate measurements, but unless you live in an area where some form of saree, lungee or shoulder cloth is normally worn, the benefits of using the vinaya measurements as a guide to do so seem limited. But there’s nothing to prevent individuals from adopting a degree of simplicity in their clothing in general, with respect also to its functional purpose.