rabbiofberlin,
It is Tosafos D”H “B’rum Shalosh Amos” to Pesachim 109b. The teshuva from R’Moshe where he says that the shiurim for etzba and ama regarding mikvaos (where the point of the shiur is to have enough water to cover an actual person) change depending on the size of the people in each generation is Yoreh Deah Ch”2, S”66.