לע"נ הרב דוד בן הרב יוסף קמנצקי ז"ל

Says the pasuk [Bamidbar 15-20]

רֵאשִׁית֙ עֲרִסֹ֣תֵכֶ֔ם חַלָּ֖ה תָּרִ֣ימוּ תְרוּמָ֑ה כִּתְרוּמַ֣ת גֹּ֔רֶן כֵּ֖ן תָּרִ֥ימוּ אֹתָֽהּ׃

The first portion of your dough, you shall separate a loaf for a gift; as in the case of the gift of the threshing floor, so shall you separate it.

Rashi:

כתרומת גורן – שלא נאמר בה שיעור, ולא כתרומת מעשר, שנאמר בה שיעור. אבל חכמים נתנו שיעור: לבעל הבית אחד מעשרים וארבעה, ולנחתום אחד מארבעים ושמנה.

כתרומת גרן AS IN THE CASE OF THE GIFT OF THE THRESHING FLOOR [תרומה גדולה] for which no minimum quantity is stated in the Torah, and not as the "Gift-offering of the Tithe (תרומת מעשר, the kohen's share in the Levi's tithe) for which a minimum quantity is stated (cf. Numbers 18:26) (Sifrei Bamidbar 110:1). — The Sages, however have stipulated a minimum quantity for it (the portion set aside) — for a private person one twenty-fourth, and for the baker one forty-eighth of the dough (Sifrei Bamidbar 110:1; Mishnah Challah 2:7).

This Rashi is very difficult to understand! There is no explicit source that teaches that תרומה has no minimum shiur. Just from the fact that it says "ראשית דגנך" without specifying how much, we assume that it has no shiur. [Look in the Sifsei Chachomim here and Rashi Avoda Zara ע"ג: ד"ה חטה]. If so, why do we need a pasuk to teach us that חלה doesn't have a shiur. It should be enough that it says "ראשית עריסותיכם" that implies any amount just like תרומה גדולה. It must be that חלה is more similar to תרומת מעשר than to תרומה גדולה and therefore without a pasuk indicating the contrary, we would compare it to תרומת מעשר that has a shiur. What is the similarity of חלה to תרומת מעשר??

It would appear that we can answer based on פסוק י"ח 

דַּבֵּר֙ אֶל־בְּנֵ֣י יִשְׂרָאֵ֔ל וְאָֽמַרְתָּ֖ אֲלֵהֶ֑ם בְּבֹֽאֲכֶם֙ אֶל־הָאָ֔רֶץ אֲשֶׁ֥ר אֲנִ֛י מֵבִ֥יא אֶתְכֶ֖ם שָֽׁמָּה:

Speak to the children of Israel and you shall say to them, When you arrive in the Land to which I am bringing you.

The Sifre says that in contrast to other מצוות התלויות בארץ the חיוב of חלה applies IMMEDIATELY upon entering Eretz Yisrael [from the pasuk והיה כי יביאך] as opposed to other obligations which only take effect after capturing the land. The Mishna [Challa 2-1] learns from the same pasuk that if the תבואה of חוץ לארץ entered EY and the flour was mixed with water [גלגול], there is an obligation in חלה מן התורה [even if it grew in חו"ל]  and so we pasken. 

It seems that these two dinim stem from  the same principal: What obligates one in חלה is NOT that the wheat grew in EY [like other מצוות התלויות בארץ] but that a Jew performed a מעשה גלגול in EY. Thus, the obligation takes effect immediately and applies even to wheat that grew in חו"ל. 

תרומת מעשר is similar insofar as it ALSO depends on the מעשה of a person and is not from Above [i.e. the growth]  - after a person gives מעשר ראשון to the Levi, the Levi must now separate תרומת מעשר. So we see that the obligation of תרומת מעשר is dependent on people just like חלה. So the hava amina is that תרומה has no shiur because its Kedusha comes from Hashem and it is given to Hashem [it says in the pasuk " 'לה "]. Thus, the amount given can't be limited on either end but rather depends on the generosity [or lack thereof] of the giver. But regarding תרומת מעשר which devolves as a result of the מעשה of a person we say that it is possible to limit the חיוב and שיעור. So too, we would think that חלה which also depends on a person would have a set shiur like תרומת מעשר. That is why we need the לימוד from the pasuk that חלה has no shiur like תרומה!     

We can now explain what Rashi says in פסוק כ"א on the words

 "ראשית עריסותיכם תתנו לה' תרומה לדרתיכם". 

Says Rashi: 

תתנו לי״י תרומה – לפי שלא שמענו שיעור חלה נאמר: תתנו, שיהא בה כדי נתינה.

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.