From: Rabbi Moshe Sofer
To: Unknown recipient
The Chasam Sofer affirms that the original gris measurement for kesamim remains fully in effect despite modern lice being smaller, since the kesamim decree was a formal rabbinic enactment that cannot be altered without a new formal decree. He supports this by developing a comprehensive theory reconciling the Kesef Mishneh, Taz, and Minchas Yaakov on the sefeik sefeika structure underlying the leniency of kesamim, concluding that stains smaller than a gris were never included in the original enactment and remain permitted.
Source: Shu"t Chasam Sofer YD §182