Size of the “Zecher l’Churban Space”
20. Modern-day measurements. The “zecher l’Churban space” must be one amah by one amah. In modern terms, that is 48x48 cm [19x19 in.] (הגר''ח נאה) or 58x58 cm [23x23 in.] (החזו''א). Since this halacha is d’Rabanan, one may l’chatchila use Rav Chaim Naeh’s smaller measurement (הליכות אבן ישראל עמ' שפ''ה).
21. Square. The area left unfinished must specifically be a 1x1 amah square, not a longer, narrower space, even if the total area is one amah square (פמ''ג משב''ז סק''א הובא בשע''ת). In a case where there is not enough room on the wall to make an amah-high square, e.g., between the top of the door and the ceiling, one may combine the space on the wall with some of the ceiling to get to the one-amah measurement and leave both without plaster (שו''ת משכנות הרועים סי' ח''י, הגר''ש קלוגר שם).
Location
22. Opposite the entrance. L’chatchila the unfinished square amah should be opposite the entrance, in keeping with the wording of the Gemara and poskim (גמ' ב''ב דף ס' ע''ב, רמב''ם פ''ה תענית הי''ב, שו''ע שם). This means on the wall facing the entrance so that one sees it right when he enters the house.
23. Over the entrance. Some leave the space over the entrance. It is not seen immediately upon entering, but it is visible to a person sitting in the room opposite the entrance. There is a reliable basis for this minhag (מנהגי חת''ס, הובא במ''ב שם סק''ג, שע''ת).
24. Side wall. If there is no option to leave an unfinished space by the entrance, it may also be left on a side wall that can be seen when entering the house or from inside the house.
25. Between two doors. If a house has two doorways at the entrance, one further in than the other with a small foyer between them, the space should be left on a wall facing the inner door, not between the two doors. This is because the area between the doors is not considered a dwelling space (הגריש''א). However, some hold it may be left there too, especially if there is nowhere else for it (הליכות אבן ישראל).
26. Stairwell. When there are multiple residential units in a building, leaving a square amah in the stairwell does not work to exempt all the residents from leaving a space in each of their apartments, as it is not in their houses.
27. Shared unit. If there was a single apartment that the landlord divided into two, and he left the unfinished square amah in its original place, which is now inside the first door but in the small foyer between the private entrance doors to each smaller unit, some hold they are yotzei the chiyuv with this, and the tenants do not need to leave a space in each smaller unit (שו''ת שבט הקהתי).
28. However, others hold since there is a separate chiyuv on every house, the small foyer is considered a shared courtyard, so they are not yotzei their chiyuv to remember the Churban there.
