The Mishnah cites, in the name of Rabbi Shimon ben Azzai, a detailed description of the Paroches: its quality, its dimensions, the cost of making it, and how it was immersed.
The description of the Paroches:
Its thickness - one tefach.
Its weave - it was woven on seventy two nimin, that is, seventy two threads. Some have the reading "nirin" - the heddles used in the weaving process, which is an alternative version of the word.
The thickness of each thread - each and every thread was made of twenty four strands.
Its dimensions - forty amos in length from top to bottom, and twenty amos in width. These are the dimensions of the Paroches that fit the entrance of the Ulam, the antechamber before the Heichal, which was forty amos high and twenty amos wide.
The cost of the Paroches:
Its cost was eighty two ribo, that is, eight hundred and twenty thousand. The Mishnah does not state which coin was meant, but presumably the reference is to dinars.
Several ways of understanding this expression have been offered:
The cost of the Paroches came to eight hundred and twenty thousand dinars.
Some have a slightly different reading in the text: not "ribo" in the sense of myriads, but "rivos" - young women. That is, eighty two young weavers took part in producing this Paroches.
A further explanation, according to the original reading: eight hundred and twenty thousand is the number of threads in the Paroches.
Two new Parochos were made every year, and three hundred kohanim would take part in immersing it in the mikvah.
Are three hundred kohanim an exaggeration?
The Gemara explains that the number three hundred is an exaggeration, and the intent is only that many kohanim were needed. This is the accepted usage in the language of Chazal: when they wish to overstate a number, they use the number three hundred, even though they mean nothing more than a large quantity.
However, there is an explanation from the Gra who calculates that if you place people's hands around a Paroches of such a size, measured in tefachim, the number three hundred kohanim indeed comes out. There was room around the Paroches for the hands of three hundred kohanim for the purpose of immersing it. According to this, the number is not an exaggeration at all: it may be that in practice three hundred kohanim were not used, but theoretically three hundred kohanim could have taken part in the immersion.
The calculation in detail:
The calculation works out according to the opinion that an amah contains five tefachim, and a tefach is the size of a man's fist.
The perimeter of the Paroches: forty amos of length on each of the two sides and twenty amos of width on each of the two sides - one hundred and twenty amos.
One hundred and twenty amos times five tefachim - six hundred tefachim.
Two tefachim for each kohen, and so the number comes out to exactly three hundred kohanim.
In summary: in the name of Rabbi Shimon ben Azzai we learned the description of the Paroches: its thickness was a tefach, it was woven on seventy two threads with twenty four strands in each thread, and its dimensions were forty by twenty amos, matching the dimensions of the entrance of the Ulam. We examined its cost, eighty two ribo, and the three ways of understanding the expression: dinars, young weavers, or the number of threads. We also learned that two were made every year, and that three hundred kohanim would immerse it - a number that the Gemara considers an exaggeration, and which according to the Gra's calculation is precise.