The Ba'al haTurim takes up the question of why the Torah juxtaposes the parsha of shoftim with the previous parsha that speaks of the moadim. Sefas Emes explains that once we've discussed the shalosh regalim at the end of last parsha, the Torah moves to the remaining holidays, the yamim noraim. Shoftim = justice and judgment, the theme of the upcoming days of Rosh haShana and Yom Kippur. Hard to believe we are almost there.
I have one and the same question on two separate comments of Rashi:
1) Rashi identifies "reishis degancha" (18:4) as terumah and then adds that m'doraysa there is no shiur for terumah, however Chazal instituted different shiurim, one of the generous person, one for the average guy, and one for the less generous. Rashi goes yet a step further and gives us the asmachta source for this din.
2) In the same pasuk Rashi identifies "resihis geiz tzoncha" as the first shearing from one's sheep, and Rashi again adds that m'doraysa there is no shiur, but Chazal instituted a shiur and Rashi explains what it is and the source.
Why does Rashi need to discuss the issue of shiur in this context? What is lacking in understanding the peshuto shel mikra if one does not have that information? Furthermore, even if Rashi needs to tell us something about the shiur, does he need to go into detail about the source, the asmachta?
I don't have an answer. The question just occurred to me when I was reviewing the parsha and I can't think of anything (yet : )