משנה על כלים 17:11

One who makes a new light hole, its minimum size is that of a hole made by the large drill of the Temple chamber. The remains of a light-hole [the size is] two fingerbreadths high by a thumb-breadth broad. The following is considered the remains of a light-hole: a window that a person had blocked up but had not been able to finish [being blocked up]. [A hole] that was bored by water, or by reptiles or eaten away by salt: the minimum size is that of a fist. If he intended to use it, its minimum size is one handbreadth square; For lighting its minimum size is that of a hole made by the drill. The holes in grating or lattice-work may be joined together to form [an opening] the size of a hole made by the drill, according to the opinion of Bet Shammai. Bet Hillel says: unless there is a hole of the size made by the drill in one place. [The above applies] for purposes of allowing the uncleanness to come in or to go out. Rabbi Shimon says: only for allowing the uncleanness to come in; but for allowing the uncleanness to go out [the minimum size] is one handbreadth square.