Ohalot, Chapter Thirteen, Mishnah Four
1) 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.
a) Bet Hillel says: one handbreadth square.
2) [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.
Section one: According to Bet Shammai, if one makes a hole in the wall to store a specific item, that hole needs to be large enough to actually store the item for it to allow impurity from one side to the other.
Bet Hillel imposes a standard sizeone handbreadth square. This is the measure we saw above in mishnah one.
Section two: If one makes a hole to look through (what we might call a peep-hole, ignoring the connotations of this word) or a hole to talk through or for any human use, the hole must be the size of a handbreadth.