TT #176 · Parshas Tazria · Aliyah 5 · Passuk 27
English: The priest shall examine him on the seventh day. If the lesion has spread in the skin, the priest shall pronounce him unclean; it is a tzaraas affliction.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:27
Torah Temimah cites Sifra (Toras Kohanim) for the minimum qualifying size of a gris.
Sifra — Vayikra 13, Midrash Sefer
English: Tzaraas is measured at the minimum qualifying size of a gris.
Textual derivation
The Sifra applies the standard minimum measure to this lesion.