(Mishnah): He must diminish it.

How does he diminish it? He makes a platform at the opening of the Mavuy and permits the Mavuy.

Question: How much [must the platform enter the Mavuy, i.e. what is the minimum width]?

Answer #1 (R. Acha citing R. Hoshayah): It must be four Tefachim, the [minimum] Shi'ur of an [important] place.

Answer #2 (R. Yosah): It must be a Tefach, the Shi'ur (minimum width) of a Korah.

Answer #3 (Talmidim): Any amount [suffices], for there is not a height of 20 Amos for the entire width of [the Korah, i.e.] a Tefach.