The Gemara in Masechet Menahot (33) establishes that the Mezuza should be placed on the upper third of the doorpost, but not within a Tefah (handbreadth) from the top of the frame. The reason is that the Torah juxtaposes the Misvot of Tefillin and Mezuza ("U’kashartam… U’chatabtam"), suggesting a parallel between the two obligations. Thus, just as the Tefillin Shel Yad is placed on the upper third of the arm, the Mezuza should similarly be placed on the upper third of doorframe.
This Halacha is codified by the Rambam (Rabbi Moshe Ben Maimon, 1135-1204), in Hilchot Mezuza (6:12), who clarifies that ideally the Mezuza should be placed precisely one-third of the way down from the top of the doorframe. Nevertheless, it may be placed higher, as long as it is lower than a Tefah from the top. The Rambam’s ruling is cited by the Bet Yosef and Shulhan Aruch (289:2).
The question arises as to whether this applies to a very high doorpost. If the door is very tall, and the Mezuza is placed in the upper third of the post, one will be unable to touch it, and in some cases it might not even be easily visible. Should the Mezuza be placed lower down the doorframe in such a case?
Tosafot (commentaries by Medieval French and German scholars), in Masechet Menahot, cite the Talmud Yerushalmi as stating that if the doorframe is high, the Mezuza should be placed at the height of one’s shoulder. However, as Tosafot note, the Talmud Bavli makes no such distinction, and thus clearly does not accept this ruling of the Yerushalmi. The Rambam and Shulhan Aruch likewise make no such mention of this exception to the rule, indicating that they, too, did not accept the Yerushalmi’s ruling. Accordingly, Hacham Ovadia Yosef rules in his Yabia Omer (vol. 2, Yoreh De’a 21) that even if the doorframe is very high, the Mezuza must be placed in the upper third. He adds that if the Mezuza was placed lower than the upper third, it must be removed and reaffixed in its proper location. Nevertheless, in the case of a tall doorframe, one does not recite a Beracha when reaffixing the Mezuza in the upper third of the frame, since according to some opinions it had been placed correctly. (This is in contrast to the case of an ordinary doorframe, where a Mezuza placed lower than the upper third must be removed and reaffixed in the upper third with a Beracha, since according to all opinions it had not been placed correctly.)
It should be mentioned that the measurements should be taken before one recites the Beracha. One should go around the house, make measurements of the doorframes and mark the upper third, before proceeding to recite the Beracha and affix the Mezuzot.
Summary: A Mezuza must be placed on the upper third of the doorframe, regardless of the height of the frame.
