Responsum of R' S.R. Hirsch
Shemesh Marpei, Orah Hayim, Siman 23
Translated from Hebrew by yitznewton; emphasis added
Tuesday of Ki Savo, 5644 (September 2, 1884)
[In response to R' Pinchas Elchanan Wechsler of Schwabach, regarding how to render Torah measurements in centimeters]
[R' Hirsch first apologizes that he was forced to limit his research and response due to the state of his health]
[...]
In my humble opinion there is no doubt that you have done according to Torah law in seeking a mean fingerbreadth between the largest and smallest man's [finger] found among you. If you establish this value to calculate the practical Torah measurements that you come upon [including] the size of the lechi of your city's eruv, there is no doubt that you have discharged your obligation, for you will have done that which is incumbent on you in Torah law. There is also no doubt, in my humble opinion, that you should not be surprised if other Rabbis elsewhere will not [have the same value as] yours, with one rather being less and another greater than your measurement.
For to my limited knowledge, it is totally impossible to definitively establish a single universal value for the measurements of the Torah's laws, and the Blessed Giver of our Torah did not have this in mind at all. Are not all the Torah's measurements in essence the dimensions of fruits and parts of the body (olive, pomegranate, barley-corn, kotevet, egg, fingerbreadth, handbreadth, cubit, etc.), and is it not so that the size of fruits and the stature of people, on which these measurements are based, differ in every place and generation? So perforce there will be varying values for the measurements of the Torah. In my humble opinion this does not reduce the truth of the Torah but rather increases it, for the goal of all of the Torah's laws is only to orient human affairs. The basis for all their measurements as given to us is, in my humble opinion, בערך מה אל ענין א' מעניני בני אדם. For example, in my humble opinion it is quite likely that the measurement given for the height of a mehitzah, whose purpose is to delineate human domains, will be larger or smaller according to the the height of the average person in a given time and place. I cannot elaborate, and anyway there is no need for an understanding person such as yourself. Also, even in a single time and place, fruits and people are not identical in size, and it is all but impossible that all who measure will find precisely the same average. [...]
[R' Hirsch goes on to recall a history of having local official shiurim, and advocates this]
(I didn't understand the bit I quoted in Hebrew; cheers to anyone who does and can translate it)