Ohalos 14:1

A projection conveys ritual impurity regardless of its size; a balcony and a canopy do so when they are have an opening of a handbreadth. By a projection we mean that which faces downward, while a balcony faces upward. They said that a projection conveys ritual impurity regardless of its size in regard to a projection that is three rows of stones above the doorway, which is 12 handbreadths (about 36”). Higher than that, a projection only conveys impurity if it has an opening of a handbreadth. Cornices and moldings convey ritual impurity when they have an opening of a handbreadth.

Ohalos 14:2

A projection above a doorway conveys impurity when it has an opening of a handbreadth. If it was above a window by two fingerwidths high or the size of a hole made by a drill, then it conveys impurity regardless of its size; Rabbi Yosi says when it’s of equal size to the window.