rabbiofberlin,
You asked “A man’s height-as per mishneh and gemoro- is three amos (see mikveh)which is 18 tefochim so, ‘baal korcho’, up to the shoulders is significantly less? R’Moshe (in chelek 3) tries to answer but I did not fully understand his answer.”
I don’t know what R’Moshe wrote. But Tosafos answers this question. The shiur of 3 amos is without a person’s head. It is still sufficient for a mikva to be 1X3 because water displacement will allow it to cover his head.
The following is from memory, but if needed I can probably find the sources: I believe R’Moshe holds that the shiurim of etzbah, tefach and ama will change based on the physiology of the current generation. In other words “etzba” is the width of the average finger, a tefach is four etzbaos and an Ama is is 6 tefachim. I recall that R’Moshe took a survey and determined that an “etzba” was 3/4 of an inch. Which would make a tefach 3 inches and an Ama 18 inches. This means that 3 amos is 4 and half feet or to the shoulders of the average person on the Lower East Side in the 50s.
I would submit that today, people are larger and the etzba is closer to an 9/10s of an inch. Therefore, modern mechitzos, while still being 18 amos tall, should have to be higher.