Bava Batra 163A

הן ואוירן, או דלמא הן ולא אוירן?

does this refer to the size of lines with the space between lines added? Or is it perhaps referring to lines of writing themselves, without their spaces?

אמר רב נחמן בר יצחק: מסתברא דהן ואוירן, דאי סלקא דעתך הן ולא אוירן ⁠– שיטה אחת בלא אוירה, למאי חזיא? אלא שמע מינה ⁠– הן ואוירן! שמע מינה.

Rav Naḥman bar Yitzḥak said: It stands to reason that it is referring to the lines with their spaces. As, if it were to enter your mind that it is referring to the lines without their spaces, for what is one line without its space fit? The baraita did not have to state that a document with a single blank line after the text, measured without counting spaces, is not forgeable; this is obvious. Rather, one may conclude from this claim that the reference is to two lines with their spaces. The Talmud affirms: Conclude from this claim that it is so.

רבי שבתי אמר משמיה דחזקיה: שני שיטין שאמרו ⁠– בכתב ידי עדים, ולא כתב ידי סופר. מאי טעמא? דכל המזייף, לאו לגבי ספרא אזיל ומזייף.

R' Shabbtai says in the name of Ḥizkiyya: With regard to the gap of two blank lines between the text and the signatures, which the rabbis said invalidates the document, the lines are measured by the handwriting of witnesses, and not by the handwriting of a scribe, who is presumably skilled enough to write in a smaller script. What is the reason for this? Anyone who forges a document, adding additional lines to the document, would not go to a scribe and ask him to forge it; he would execute the forgery himself, or have another unscrupulous person who is not a professional scribe forge it. Therefore, in order to present a concern for possible forgery, a document must have two blank lines that are measured by the handwriting of an ordinary person, such as one of the witnesses.