Ohalot, Chapter Fourteen, Mishnah Two
1) A projection that is above a doorway forms a passage for the uncleanness when it is one handbreadth wide;
2) If above a window two fingerbreadths high or the size of a hole made by a drill, when of any width whatsoever.
a) Rabbi Yose says: when of equal size [to the particular window].
Section one: In yesterday’s mishnah, we learned that a projection above a doorway brings in impurity no matter how big the projection is. Today’s mishnah teaches that the doorway must be at least one handbreadth wide to allow the impurity to enter the house.
Section two: If the projection is above the type of window that needs to only be two fingerbreadths high to allow in impurity (this is the remnant of a light-hole, see 13:1) or one that needs to be only the size of a drill (see also 13:1), it (the projection) brings in the impurity no matter how small it is. According to this opinion, the projection will offer protection to such a small window, even if the projection is of very small size.
Rabbi Yose says that the projection must be as large as the window that it protects. So if it protects a two fingerbreadths size window, the projection must be two fingerbreadths in order to bring impurity into the house.