Why does the Gemara consider Rav Chanin's statement to be a contradiction to
Rav's statement? Perhaps Rav meant that the *other* Shi'urim, which are not
dependent on Chitah, Se'orah etc., are learned from a Halachah l'Moshe
mi'Sinai? For instance, it is learned from a Halachah l'Moshe mi'Sinai that
the Shi'ur of an Ohel (for spreading Tum'ah) is a cubic Tefach, as Rashi
himself tells us (Berachos 19b DH Devar Torah, Sukah 4a DH Hachi Garsinan),
and it is learned from a Halachah l'Moshe mi'Sinai how long one must stay in
the Azarah b'Tum'ah to be punishable with Kares (RASHI Shavuos 14b DH
Chayav). Many other Shi'urim must also have been learned from a Halachah
l'Moshe mi'Sinai -- for instance the smallest amount of Sheretz that is
Metamei is k'Adashah; the size of a Nega Tzara'as is a Gris; two black hairs
invalidate a Parah Adumah, etc.! (MAHARATZ CHAYOS; ARUCH LA'NER)
(a) RASHI (DH Shi'urin) may be answering this question by adding the words,
"Shel Isurin." The Gemara is assuming that Rav's Halachah l'Moshe mi'Sinai
is identical to Rebbi Yochanan's Halachah l'Moshe mi'Sinai in Yoma 80a.
Rebbi Yochanan made a point of saying that "Shi'urim *Shel Onshin*" are
Halachah l'Moshe mi'Sinai, i.e. the Shi'ur for which a person is punishable
when transgressing a prohibition (that involves an object). Why did Rebbi
Yochanan add the words "Shel Onshin" (as the RASHASH asks, there)? Perhaps
he meant to emphasize that even the Shi'urim for transgressions, which Rav
Chanin learns (among other Shi'urim) from "Eretz Chitah...," are Halachah
l'Moshe mi'Sinai. The other Shi'urim that we mentioned in our question, are
not related to an object mentioned in a Torah prohibition but to general
laws of Tum'ah etc.
If so, Rav, too, means to say that the Shi'urim of Onshin (or as Rashi puts
it, of Isurin, transgressions), are Halachah l'Moshe mi'Sinai, in which case
he is referring to those very Shi'urim that Rav Chanin learns from the
verse. (M. Kornfeld - however, Rashi Eruvin 4a, in a parallel Sugya, does
not mention the word "Isurim")
(b) Alternatively since Rav didn't qualify his statement by limiting it to
specific Shi'urim, it seems that he meant to make a *general* statement
about Shi'urim. If so, the Gemara is asking that since there are *some*
Shi'urim which are indeed learned from a verse, how could Rav make a blanket
statement that all Shi'urim are Halachah l'Moshe mi'Sinai?
This will answer some other questions that may be asked on our Sugya as
well. The ARUCH LA'NER asks, why does the Gemara challenge Rav by saying
(6a) that Chatzitzin are learned from a verse? There are laws of Chatzitzah
with regard to performing Avodah in the Beis ha'Mikdash too, and some of
them (such as whether Tefilin is considered a Chatzitzah, Zevachim 19a and
Tosfos there) seem to have their sources in Halachah l'Moshe mi'Sinai.
Perhaps these are the Chatzitzin that Rav was referring to! Similarly, why
does the Gemara ask which laws of Mechitzin are Halachah l'Moshe mi'Sinai?
Even if we learn the height of the Mechitzah from a verse, the Halachah
l'Moshe mi'Sinai is necessary to teach that the third wall of a Sukah need
only be a Tefach long, as the Gemara itself tells us on 6b! (ARUCH LA'NER,
SEFAS EMES, MAHARATZ CHAYOS, RASHASH -- all of whom suggest forced answers)
According to what we have said above, the Gemara could not have suggested
such answers. If the Halachah l'Moshe mi'Sinai is teaching one specific law
of Chatzitzah (i.e. whether or not Tefilin or Chotzetz) or of Mechitzah
(i.e. that the third wall of a Sukah need only be a Tefach), Rav would not
have made a blanket statement that "Chatzitzin u'Mechitzin" are Halachah
l'Moshe mi'Sinai. Instead, he must have meant that some *general*, universal
Halachos of Chatzitzah ("Rubo u'Makpid Alav") and of Mechitzah ("Gud
v'Lavud...") are Halachah l'Moshe mi'Sinai. (M. Kornfeld)