"Azul Rabbanan batar deshchicha"
We are talking about chiyuv chatat; does it mean that the Torah gave the Chachamim power to decide what is a shiur chashuv (similar to melachot assurot bechol hamoed), or that the Gemarah is explaining how they reached that conclusion?
Avraham Djmal, S.Paulo,Brazil
There is an argument among the Rishonim regarding exactly what we learn from the Mishkan with regards to Shabbos.
The main opinion seems to be that of many Rishonim, such as the RITVA and CHIDUSHEI HARAN, who understand that only the types of Melachah done in the building of the Mishkan are forbidden on Shabbos. However, the paramaters (i.e. what type of tying, what type of building) of the Melachah are in fact set up by Chazal. This is also the opinion of the TESHUVOS HAREMAH (#119). This means that Chazal decided, as you explained in your first possibility, what is a Shiur Chashuv to make someone Chayav for Meleches Shabbos.
There are other opinions, such as that of the AVNEI NEZER (#211) in the name of the ROSH that the amounts of the Melachah were also learned from the Meleches Hamishkan. However, it is possible that he too will agree that whenever Chazal did not see a clear indicator for the minimum shiur of a Melachah from the Mishkan, this means that the Torah left it to Chazal to determine the Shiur.
We find a similar concept, where the Torah left it for Chazal to determine the Melachah, in Moed Katan 17b regarding the Melachos of Chol ha'Moed, "Lo Masrecha ha'Kasuv Ela la'Chachamim." This theme is repeated in many places in the words of Chazal and the Rishonim.
As I understand it, this means that the Torah felt confident that the Chachamim would understand the finer intricacies of the Halachah in its motives, and thus they would know exactly what the Torah wanted from us in these situations.