Q) Where does the minhag of writing an 11 line megillas Esther come from?
A) The Shulchan Aruch (Orach Chayim 691:4) tells us that when writing the aseres bnei homon in the megilla, one should ensure that the word “ish” is written at the beginning of the daf, and “aseres” at the end. Because of this, the predominant minhag is to write the aseres bnei homon in a column by itself, using large letters to enable “ish” to be written at the beginning of the daf, and “aseres” at the end.
The Vilna Gaon (Bi’ur Ha’gro on Shulchan Aruch ibid, see also Ma’ase Rav 243) has a different interpretation of the Gemoro upon which this halocho is based on. He explains that the requirement is to write “ish” at the beginning of the line and “aseres” at the end of the line (not column). According to the Vilna Gaon there is no reason or requirement to write the aseres bnei homon in their own column. Furthermore, one should not write the aseres bnei homon in their own column, since in a regular megilla (that consists of more than 11 lines) one will be forced to use large letters in order to fill the column – and there is no mesorah to write the aseres bnei homon in large letters.
Therefore, those who wish to maintain the halacha brought down in Shulchan Aruch (which is also the longstanding minhag) of writing the aseres bnei homon in a column by itself, and also to be choshesh for the view of the Vilna Gaon, write an 11 line megillah so that the aseres bnei homon can be written in their own column – in normal sized letters.