方丈  Literally a square zhang , which is one hundred square Chinese feet. (1) The front room, or parlor room of a monastery. (2) The room of the head monk. This meaning is originally derived from the story of Vimalakīrti's 維摩 room, which was supposedly one square zhang (about 9 square meters). It later becomes a term of respect used to address a teacher or a superior monk.

An abbot, 寺主 head of a monastery; the term is said to arise from the ten-foot cubic dwelling in which 維摩 Vimalakirti lived, but here seems to be no Sanskrit equivalent.

A Dictionary of Chinese Buddhist Terms, William Edward Soothill and Lewis Hodous

一丈四方之室。又作方丈室、丈室。即禪寺中住持之居室或客殿，亦稱函丈、正堂、堂頭。印度之僧房多以方一丈為制，維摩禪室亦依此制，遂有方一丈之說；轉而指住持之居室。今轉申為禪林住持，或對師父之尊稱。俗稱「方丈」或「方丈和尚」。〔維摩經文疏卷二十二、法苑珠林卷二十九、大唐西域求高僧傳卷上慧輪傳〕

佛光電子大辭典

p1432

寺廟的住持和尚。相傳維摩居士所住的石室，長寬祇有一丈，方丈之名，即是由此而來。

陳義孝編, 竺摩法師鑑定, 《佛學常見辭彙》

（堂塔）禪林之正寢，住持之住所也，故稱寺主曰方丈，因其住於此也。古來之說維摩居士之石室，四方有一丈，丈室之名，始基於此。法苑珠林感通篇曰：「吠舍釐國宮城周五里，宮正北六里有寺塔，是說維摩經處。寺東北四里許有塔，是維摩故宅基，尚多靈神。其舍疊磚，傳云：積石，即是說法現疾處也。於大唐顯慶年中敕使長吏王玄策，因向印度過淨名宅，以笏量基，止有十笏，故號方丈之室也。」傳燈錄禪門規式曰：「長老既為化主，即處於方丈，同淨名之室，非私寢之室也。」案方丈之名，先唐王玄策已有其說。王簡栖頭陀寺碑文曰：「宋大明五年始立方丈茅茨。」注，高誘曰：「堵長一丈，高一丈，面環一堵，為方丈。銑曰：宋孝武皇帝時也，言立方丈之室，覆以茅茨之草，以置經家也。」則方丈唯言僧坊之狹小而已，必非由於維摩也。曇鸞之往生論註上曰：「如維摩方丈，苞容有餘，何必國號無貲，乃稱廣大？」

丁福保《佛學大辭典》