Q. How big does a sefer have to be for the purpose of this mitzvah? (writing a sefer on the internet- see prior question 1000)

A. The Talmud Shabbos (115b) teaches that an old Sefer Torah that has a minimum of eighty five surviving kosher letters should be saved on Shabbos from being destroyed by fire. Also, the minimum size for a Sefer Torah to contaminate with Tumah the hands that touch it is eighty five letters (Yodaim 4: 5.) The above is deduced from the small parsha of “Binsoa Ha’aron” (Bamidbar 10:35) that is considered as a complete sefer.
Horav Shlomo Miller’s Shlit”a opinion is similar.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a.