Related על מדות 4:5

One who makes a place for a rod, for tongs, or a lamp, the minimum size is whatever is needful, according to the word of Bet Shammai. Bet Hillel says: one handbreadth square. [If it was made] for a peep-hole, for speaking through to his fellow, or for [human] use, the minimum size is one handbreadth square.