We can now explain what Rashi says in פסוק כ"א on the words
"ראשית עריסותיכם תתנו לה' תרומה לדרתיכם".
Says Rashi:
YOU SHALL GIVE TO HASHEM TERUMAH – Because we have not heard any mention of a definite minimum quantity for the Challah it is stated here "you shall give", implying that it shall contain sufficient to constitute a "gift" (Sifrei Bamidbar 110:2).
The Mizrachi already wondered that it is explicit in the earlier pasuk that there is no shiur like Terumah where there is no shiur, so how can Rashi says that there is a shiur of "כדי נתינה"?
It is well known what the Noda Bi-Yehuda [יו"ד ר"א עיי"ש] says that there are 2 stages. Regarding the separation [הפרשה] which permits the טבל we say that it is כתרומת גורן and there is no shiur. But with respect to fulfillment of the mitzva of נתינה, one has to give "כדי נתינה" and if he gave less than this then he has permitted the טבל but not fulfilled the mitzva. The Gur Aryeh writes that the shiur of כדי נתינה is not מעכב but there is a מצוה מן התורה. It emerges that חלה DOES have a shiur unlike תרומה that has no shiur even for the נתינה. We have have to understand why.
We can explain according to what we wrote earlier that תרומה is totally מעשי שמים. Therefore there is no shiur for the הפרשה which creates the שם תרומה or for the נתינה which is a mitzva on the מציאות of these fruits to give them to the kohen. But חלה is a dichotomy. On one hand the מציאות of חלה is created by man when he mixes the flour and water [גלגול]. However the חלות שם of עיסה, that it should be halachically considered dough that is obligated in separation, is from שמים. Man doesn't have the power to create that - only Hashem does. Therefore, when it comes to הפרשה which creates the חלות שם חלה there is no shiur, since the שם הדבר is created from שמים [as we saw regarding תרומה, what comes from שמים can't be limited by a shiur]. But the נתינה which relates to the מציאות of the physical reality of the חלה created by man has a shiur just like תרומת מעשר that has a shiur since it is man-made.
We see this from 'פסוק כ that says "ראשית עריסותכם חלה תרימו תרומה כתרומת גרן" while פסוק כ"א says "מראשית עריסתוכם" and omits the word חלה. Based on what we wrote, 'פסוק כ is telling us that חלה is מעשה שמים because the result of עריסותיכם [dough] which is חלה [as Rashi says] is not your own doing but that of Hashem and therefore there is no shiur just like תרומה גדולה [as the pasuk compares them]. While פסוק כ"א teaches that it also has a din [of having a shiur נתינה] that comes because of his own מעשה. The מעשה is that it is עריסותיכם - physical dough and not חלה and therefore the word חלה is omitted.
