A legal system requires measures. In common law, for example, there are limits on the amounts that can be claimed in a small court case, or of a time period after which a claim may be filed (statute of limitations), or a cap on monetary awards in certain cases. A law system cannot exist without limits.
In Torah law, limits are called shiurim. Eating chomtz on Pesach has a shiur of kazais for the penalty of kares(extirpation) and any amount for being prohibited. A dead sheretz(crawling creature) larger than a bean (adasha) causes impurity but smaller than an adasha does not cause impurity. One has to say a blessing after eating an olive size of food and so on.
Torah usually does not tell is what these shiurim are. It often does for time: how long does take for a sacrifice to become pasul, how long to celebrate Passover, at what time to bring Omer. With the exception of the measurements of Temple vessels, it almost never tells us how large or small something has to be, or what price a sacrifice should be. There are exceptions: Asham of 2 shekalim, thirty shekalim for one whose ox kills a slave, erachin and others, but usually not.
Yet, for practically everything, we know that there is a shiur. Where do the shiurim come form?
There are three possiblities. They could be entirely rabbinic, either completely rabbinic or rabbinic but with a Biblical sanction for rabbis to define shiurim. They could also be halacha l'Moshe Misinai(Sinaitic ir origin and orally transmitted since Sinai) or they could be derived through drasha.
The discussion in Bavli considers these possiblities in Sukkah 5b, Eruvin 4a and Brochos 41b, Pesachim 80a. In Brochos the conclusion seems to be that shiurim are rabbinic and in the other places that they are Sinaitic but attached to a verse by asmakhta. Here is the quote from Brochos.
ואידך הני שיעורין בהדיא מי כתיבי אלא מדרבנן וקרא אסמכתא בעלמא
Of note, in Yerushalmi, it is explicit that whether shiurim are rabbinic or Sinaitic is a disagrement of R. Yochanan and R. Oshiah.
דמר ר' יוחנן מעה כסף שתי כסף דבר תורה תנא רבי יוסי קומי רבי יוחנן ראיה כל שהוא חכמים הן שאמרו מעה כסף שתי כסף א"ל ויש כעין זו א"ר יונה וכל השיעורים לא חכמים הן שנתנו כזית מן המת כזית מן הנבלה וכעדשה מן השרץ
In fact, R. Oshiah advised that one should write down his tansgression and the exact amount with which they were committed, since there is no Temple and they cannot be atoned for immediately, and in some future time Rabbis can changed the size of shiurim and require a recalculation of what the sinner must bring. This is also found in Bavli Yoma 80a, where it is explained that they may say that instead of an average olive, we should use a large olive. For some reason this is not cosnidered a violation of the principle that a subsequent Beis Din cannot overturn the rule of a prior Beis Din, unless the later Beis Din is greater. But Bavli does nto connect it to a view the shiurim are Rabbinic in roigin.
Yerushalmi Peah:
אמר רבי יוסי בר בון רבי יוחנן כדעתיה דרבי יוחנן אמר כל השיעורים הלכה למשה מסיני דו אמר מעה כסף שתי כסף דבר תורה רבי הושעיא כדעתיה דרבי הושעיא אמר האוכל איסור בזמן הזה צריך לרשום עליו את השיעור שמא יעמוד ב"ד אחר וישנה עליו את השיעורים ויהי' יודע מאיזה שיעור אכל
It seems that R. Oshiah understands shiurim to be completely rabbinic, not that Torah gave Rabbis the authority to define shiur(as Gro says in his commentary to this yerushalmi), for had that been the case, a subsequent court cannot change shiurim once they were defined.
That Torah gave the rabbis the authority to define various laws, is already found in the Rishonim;this view was enunciated again and again in our day by the Chazon Ish (See Chazon Ish E'H 22,3; Y"D 5:3, Ch'M Nezikin 11:1, 8:1, Kovetz Inyanim pp. 194-197).
The hashkafic significance of the view the shiurim are rabbinic is,of course, great. It is surprising thate some of the Rabbis, certainly according to Yerushalmi, saw such a central and crucial concept of law as being rabbinic in origin.
Perhaps, it is because, as important as halacha is, it is just not as important as moral teachings, so it was left to Rabbis. (Bereshit Rabbah 60,11):"Said R. Aha: The table-talk of the servants of the patriarchs' households is more notable (literally: beautiful) than the scripture (Torah) of their descendants. Eliezer's story is recorded and recapitulated, taking up to three pages, whereas one of the fundamental rulings of the Torah, to the effect that the blood of a creeping thing defiles in the same way as its flesh, is only known to us through the superfluity of one letter in the Scriptures."
Here is another example:
Genesis Rabbah, Parashah 44
"The Mitzvot were given only to purify humanity. For what difference does it make to the Holy One, blessed be He, if one slaughters an animal at the throat or at the nape of the neck? Lo, the sole purpose is to purify humanity."
See also the distinciton in Moreh Nevukhim 3:28 between necessary laws and true ones.
Aruch L'Ner in Sukkah posits that there are two different kinds of shiurim. As I understand him, and there are other ways to understand what he says, there are logical measures. Everyone measures a meal as half a laof of bread, everyone recognizes that an olive is a minimum kind of "eating". Torah did not have to legislate these concepts and rabbis simply formalized them. I would say that ll that Rabbis did is say that "Botla Daato etzel kol adam". Once Rabbis defined these shiurim no individual could claim that his meal takes longer or that he is sated with something less than grogeres. On the other hand, in purity and impurity there is no such consensus. Why should a sheretz cause impurity above a size of a bean? Why not a hair or a stone? These "illogical" shiurim were received from Sinai. When Yerushalmi reports a view that shiurim are rabbinic, it only discussed the former kind.
The elephant in the room is the mishna that explicitly says that shiurim were made by rabbis. Aruch L'Ner implies that these mishnayos in Keilim 17 speak of "logical shiurim" but that is far from obvious. Here is one of the representative mishna from Keilim: "ח כזית שאמרו--לא גדול ולא קטן, אלא בינוני: זה אגורי. כשעורה שאמרו--לא גדולה ולא קטנה, אלא בינונית: זו מדברית. כעדשה שאמרו--לא גדולה ולא קטנה, אלא בינונית: זו מצרית. כל המיטלטלין מביאין את הטומאה בעובי המרדע--לא גדול ולא קטן, אלא בינוני. איזה הוא בינוני, כל שהיקפו טפח"
There are other indications that a later Beit Din can change an etablished halacha. An example is what Ruth Rabbah says to explain why Ploni Almoni was afraid to marry Ruth (4:6) "lest I destroy my inheritance".He was afraid that a subsequent Cort will darshin that the prohibiton to marry a Moabite includes also Moabite women. Yevamot
76b-77a reports that a debate broke out about David's fitness for the throne, when Doeg the asked why are you debating is fitness to be king, when it is not even clear that he is fit to marry into he nation, since he is a descendant of Ruth the Moabite. Whereupon the
answer was given Amoni v'lo Amonit, Moavi v'lo Moavit. To which Doeg
replied, if so why not Mitzri, v'lo Mitzrit and mamzer v'lo mamzeret? They
replied mamzer means mei am zar, which includes males and females
and then concerning amon and moav they cited the verse "since they did
not provide you with bread and water..." But Doeg even had an answer
for that verse, the men should have provided bread and water to the
men and the women should have provided bread and water to the
women. The gemara says that they were silent, unable to counter Doeg.
Doeg's objection was not refuted until Itra came and threatened to kill
anyone who rejected the drasha Moavi v'lo Moavit which he claimed to
have heard directly from the beit din of Shmuel ha-ramati.
Rambam in Mamrim 2:1 that the beit din ha-gadol has he authority to change the halachah of a previous beit din based on its own understanding of the
relevant Biblical verses. See also here
You see here that this license does not extend to a Halacha L'Moshe MiSinai.
Form this passage in Yerushalmi you can see that R. Oshiah, who holds that shiurim are rabbinic, says that the same way a drasha can be change, so can a future Beis Din re-legislate the shiurim. R. Yochanan, who holds that shiurim are Sinaitic, has no concern that a subsequent beit din will change the shiurim.
You might say, who would even think of changing Sinaitic Laws. However, that is another discussion.