Since B"h we are getting into mezuza laws, I would ask does anybody know about a specific lubavitch custom in regard to the shiur parsha in mezuza.
The kuntres acharon of SA Harav [32:12] writes: ובמזוזה אין צריך להיות הריוח רק כשיעור ג' יודין סוף פרשת שמע וכשיעור ג' יודין קודם התחלת פ' והיה אם שמע.
It seems there is a printing error and it should be ט' יודין as the minimum shiyur (not ג' יודין). Meaning that the Alter Rebbe didn’t require a full 3 x Asher in mezuza (as in tefilin). The difference the Alter Rebbe makes between tfilin and mezuza, can easily be explained.
This was to my knowledge the custom followed by all lubavich sofrim that I saw or heard – to leave at least 9 yudim [b'revach – well spaced] in the mezuza both before vhaya, and after shma.
Lately I heard a suggestion that this isn't a mistake, rather the Alter rebbe did instruct to leave less than a shiyur. I doubt this rumor has any reliable source, (also it doesn’t fit with the wording אין צריך להיות which means "it isn't needed" – not that it shouldn’t be done) – Does anyone know/heard something about this issue.
For pilpul the issue is quite interesting – but hoira'a l'maase to be meikal against a simple psak in SA YD 288:13, and Shach there – we must have a reliable source/psak. Note the Alter Rebbe's note in his Mare Makom on his SA 32:50 (in the new edition it is note תס) וכן משמע בגמ' ופוסקים גבי מזוזה which seems [b'pshat] to accept this as halacha.