Tractate Kelim - Chapter 27 - Mishnah 2 כלים פרק כ״ז משנה ב׳

Cloth is susceptible to midras uncleanness when it is three handbreadths by three handbreadths, and to corpse uncleanness when it is three fingerbreadths by three fingerbreadths. Sack-cloth when it is four handbreadths by four handbreadths. Leather, five handbreadths by five handbreadths. And matting, six handbreadths by six handbreadths. [All of these] are equally susceptible to both midras and corpse uncleanness. Rabbi Meir says: what remains of sack-cloth is susceptible to uncleanness if it is four handbreadths, but when in its first condition [it becomes susceptible only after its manufacture] is completed.