יורה דעה ח׳Yoreh De'ah 8

Se’if 1א׳

כַּמָּה הוּא אֹרֶךְ הַסַכִּין שֶׁשּׁוֹחֵט בּוֹ, כָּל שֶׁהוּא; רַק שֶׁלֹּא יְהֵא דָּבָר דַּק שֶׁנּוֹקֵב וְאֵינוֹ שׁוֹחֵט, כְּמוֹ רֹאשׁ הָאִזְמֵל הַקָּטָן וְכַיּוֹצֵא בּוֹ; וּבְמַחַט, אֲפִלּוּ הוּא רָחָב קְצָת כְּאוֹתָן שֶׁל רַצְעָנִים שֶׁחוֹתְכִין בּוֹ הַחוּט, אֵין שׁוֹחֲטִין בּוֹ (טוּר). וְכֵיוָן דְּלָא יָדְעִינָן שִׁעוּרָא, הַשּׁוֹחֵט בְּסַכִּין קְטַנָּה צָרִיךְ לִזָּהֵר וּלְשַׁעֵר לְפִי אֹמֶד דַּעְתּוֹ שֶׁכְּשֶׁיּוֹלִיךְ וְיָבִיא בָּהּ שֶׁלֹּא יִדְרֹס; אֲבָל בִּקְטַנָּה יוֹתֵר מִדַּאי לֹא יִשְׁחֹט. הַגָּה: וּמִי שֶׁלֹּא יוּכַל לְשַׁעֵר, יִקַּח סַכִּין כִּמְלֹא אֹרֶךְ ב' צַוָּארִין שֶׁל אוֹתוֹ דָּבָר שֶׁשּׁוֹחֲטִין (מָרְדְּכַי בְּשֵׁם ר"ח) ; וְיֵשׁ אוֹמְרִים לְשַׁעֵר בְּי"ד אֶצְבָּעוֹת, וְרֶמֶז לָזֶה, שֶׁנֶּאֱמַר: וּשְׁחַטְתֶּם בָּזֶ"ה (שְׁמוּאֵל א, יד, לד), מִנְיַן י"ד. (שָׁם וּשְׁחִיטוֹת יְשָׁנִים).

How much is the length of the knife with which he slaughters? Any amount at all; only that it should not be a thin thing that bores a hole and does not slaughter, such as the tip of the small lancet and the like of it. And with a needle, even though it is somewhat broad, like those of the cobblers with which they cut the thread, one may not slaughter with it (Tur). And since we do not know the measure, one who slaughters with a small knife must be careful and estimate according to his own judgement, so that when he carries it forward and brings it back with it he shall not press it down; but with one that is too small he should not slaughter. Hagah [Rema]: And one who is not able to estimate should take a knife of the fulness of the length of two necks of that thing which they are slaughtering (Mordechai in the name of Rabbeinu Chananel); and there are those who say to estimate by fourteen fingerbreadths. And a hint of this is in what is said, "and you shall slaughter with this" (I Samuel 14:34), bazeh, whose number is fourteen. (ibid., and old works on slaughter)