Ohalos13: 4
הָעוֹשֶׂה מָקוֹם לַקָּנֶה, וְלָאַסְפְּתִי, וְלַנֵּר — שִׁעוּרוֹ כָּל שֶׁהוּא, כְּדִבְרֵי בֵית שַׁמַּאי; בֵּית הִלֵּל אוֹמְרִים: בְּפוֹתֵחַ טֶפַח. לָזוּן אֶת עֵינָיו, וּלְדַבֵּר עִם חֲבֵרוֹ, וּלְתַשְׁמִישׁ — בְּפוֹתֵחַ טֶפַח.
There is no minimum size [for a hole] that is made for a stick, a ‘‘sword,’’ or a lamp, according to Beis Shammai. Beis Hillel say: A square handbreadth. [If a hole was made] to be a peephole, to speak with his friend, or for other usage, the minimum size is a square handbreadth.