The mishna lists the minimal size of all skin nega’im is the size of a “griss HaKalki”., the “kalkilian groat” aka “Cilician bean”. This measurement is important since it is the standard minimal size of any and all skin tzoraath variations.
So just how big is that? in short, the area covered by 36 hairs. Another description of this size is given by the rambam1)1; a חצי גרגר של פול; one half of a fava bean.
Now we could get onto exact millimeter proportion if how large a circumference that is, but that defeats the purpose of the abstract form of measurements quoted by chazal, as the Torah wants the size analysis to be up to the viewing kohen לכל מראה עיני הכהן and his good judgement based on the “flexible” units of measurement cited above and not those of the common yardstick. (Ex. Menachos 87b)
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