Reference for Kelim 27:2
הַבֶּגֶד מִטַּמֵּא מִשּׁוּם שְׁלֹשָׁה עַל שְׁלֹשָׁה, לְמִדְרָס, וּמִשּׁוּם שָׁלֹשׁ עַל שָׁלֹשׁ לִטְמֵא מֵת. הַשַּׂק אַרְבָּעָה עַל אַרְבָּעָה, הָעוֹר חֲמִשָּׁה עַל חֲמִשָּׁה, מַפָּץ שִׁשָּׁה עַל שִׁשָּׁה, שָׁוִין לְמִדְרָס וְלִטְמֵא מֵת. רַבִּי מֵאִיר אוֹמֵר, הַשַּׂק שְׁיָרָיו אַרְבָּעָה, וּתְחִלָּתוֹ מִשֶּׁיִּגָּמֵר:
Clothing may be rendered impure as a [piece which is] three by three [handbreadths] with <i>midras</i> impurity [a type of impurity due to being sat on by certain impure individuals, rendering something an Origin of impurity], and at three by three [fingerbreadths] with corpse impurity. Sack-cloth [may be rendered impure] at four by four [handbreadths]; Leather, five by five; matting, six by six; these are all equally [susceptible to being rendered impure] with <i>midras</i> and corpse impurity. Rabbi Meir says: regarding sack-cloth, its scraps [may be rendered impure] at four [by four handbreadths], but initially [it can be rendered impure only] once it is completed.