The Torah lists the dimensions of the aron as 2.5 cubits long, 1.5 cubits wide, and 1.5 cubits tall (Shemot 25:10, 37:1). The kaporet, the gold covering over the ark, is listed as having the same width and length as the aron, as we would expect, but the Torah makes no mention of its height. This issue is addressed by the Gemara in Masekhet Sukka (5a), where it asserts that the kaporet was a tefach (handbreadth) high, thus bringing the entire height of the aron to a total of ten tefachim (as a cubit consists of six tefachim).
The Gemara reaches this conclusion by invoking the principle of “tafasta mu’at tafasta,” which means that absent a clear indication of an amount (or, in this case, a measurement), we assume the smallest amount we can. In this instance, the Gemara reasons, we should assume that if the Torah did not specify the height of the kaporet, it must have been the same height as the smallest item in the Mishkan described in the Torah. This is the misgeret, the frame that surrounded the shulchan, which extended one tefach from the shulchan (25:25, 37:12). As the height of the misgeret – the smallest dimension of any of the Mishkan’s furnishings – is listed as one tefach, the Gemara concluded that the kaporet, too, must have been one tefach tall.
The Gemara then notes that there was another item mentioned in the Torah in this context with dimensions even smaller than those of the misgeret. The tzitz, the gold plate worn by the kohen gadol on his forehead, is described in a berayta as having been very narrow – just two finger-widths. This is smaller than a tefach, and so perhaps we should conclude that the kaporet was only this height. The Gemara dismisses this argument, noting that the Torah never specified the dimensions of the tzitz. In looking for a model of the kaporet’s height, the Gemara asserts, we should look to an item whose dimensions are specified by the Torah, and thus the tzitz does not provide an instructive precedent.
Tosefot explain the Gemara to mean that the measurement of two finger-widths was stated by Chazal, and does not constitute a Torah requirement. Chazal arrived at this conclusion based on the Torah’s description of the tzitz as being worn on the kohen gadol’s forehead. Since the average size of a forehead is two finger-widths, Tosefot write, it was determined that this should be the size of the tzitz. (Tosefot then proceed to cite the view of the Riva, who disagreed and maintained that the tzitz was worn on the head, and not on the forehead, as we discussed yesterday.)
Tosefot’s comment can be understood in three different ways. One possibility is to explain that when they speak of this required measurement as applying on the level of mi-de’rabbanan (rabbinic enactment), they mean that it was derived through the process of rabbinic exegesis. After all, as Tosefot immediately proceed to explain, Chazal arrived at this measurement based on their interpretation of the verse. Seemingly, then, this measurement is actually required on the level of Torah obligation, and Tosefot mean to say that it was derived through the process of the Tora she-be’al peh (the Oral Tradition), as opposed to being explicated in the verse.
Another possibility, suggested by the Mabit in his Kiryat Sefer commentary to the Rambam’s Mishneh Torah (Hilkhot Kelei Ha-mikdash, chapter 9), is that Tosefot here indeed distinguish between two different levels of obligation. On the level of Torah law, the tzitz’s required measurement is the size needed to cover the kohen gadol’s forehead; when the Torah required the kohen gadol to wear the tzitz, it established that the tzitz must be the size of the forehead of the kohen who wore it. Chazal, however, established a uniform size – two finger-widths, a measurement which they assessed was the average size of a forehead.
One might argue, however, that neither of these two readings account for the Gemara’s formulation that “the Torah gave no measurement at all” for the tzitz (“lo natena Torah…mida kelal”). According to both readings, the Torah did, in fact, assign a measurement for the tzitz, contrary to the straightforward implication of the Gemara. Therefore, Rav Yitzchak Zev Diskin suggests in his Zivchei Tzedek a subtle distinction between a formal required measurement and the obligation cast upon the kohen. According to Tosafot, Torah law does not assign a particular measurement, but requires the kohen to cover his forehead with the tzitz. This requirement relates to the kohen, not to the validity of the tzitz as a priestly garment. And thus the literal meaning of the Gemara’s comment is correct – the Torah did not give any particular measurement for the tzitz, and thus it cannot serve as a model for the kaporet. The tzitz inherently does not require any particular measurement, though the kohen is required to wear a tzitz that covers his forehead.