The Gemora asks: And how much is the space that disqualifies a document?
Rav Yitzchak ben Elozar said: As much space as would be required for the writing of “lecha, lecha” above each other. [The “lamed” is a tall letter, and that should not touch the “end of chaf,” whose leg hangs low.] This indicates that he is of the opinion that the limit is two written lines and four blank spaces (for each of those words needs a blank space above for the “lamed” and a blank space below it for the “end of chaf”).
Rav Chiya bar Ammi said in the name of Ulla: As much space as would be required for the writing of a “lamed” on the top line and an “end of chaf” on the lower line. This indicates that he is of the opinion that the limit is two written lines and three blank spaces (for there is no “lamed” on the bottom line, and there is no “end of chaf” on the upper one).
Rabbi Avahu said: As much space as would be required for the writing of “Baruch ben Levi” on one line. Evidently he holds that the limit is one written line and two blank spaces (one space above for the “lamed,” and one space below for the “end of chaf”).
The commentators ask on Rabbi Avahu’s opinion: How can he maintain that the document is invalidated with one line of text and two spaces (one above and one below it) when the Gemora above clearly ruled that it is a space of two lines of text that invalidate a document!?
Some Rishonim answer that Rabbi Avahu disagrees with Chizkiyah and hold that the amount of space needed to disqualify the document is not measured according to the handwriting of the witnesses; rather, it is based upon the handwriting of the scribes. Accordingly, it is sufficient with one line of text and two empty spaces, for the scribe may manage to use the space for two lines of text with an empty space between them. According to this explanation, Rabbi Avahu agrees with the braisa above, which ruled that a space of two lines of text invalidates a document.
Tosfos Ri”d answers that Rabbi Avahu maintains that the space of two lines of text which invalidate a document is without any space above them or below them; it is referring just to the lines of text and the space between them (not like Rav Nachman bar Yitzchak). According to this interpretation, Rabbi Avahu’s amount and the ruling of the braisa are precisely identical, for one line of text with two spaces is exactly the same as two lines of text with one space between them.
The Ritv”a writes that Rabbi Avahu holds that the two lines mentioned above that invalidate a document refer to two blank spaces surrounding one line of text.