batim 4x4 and placement on the arm

According to the SA Harav (and pashtus from the Shimusha Rabbah himself and also sefer Hatruma, Eshkol, Pri Megadim) the bayis shel yad need not be 2 etzbaos, which according to SA Harav refers to the ketzitzah (hence ketzitza is 4x4). Even according to those that it applies to the yad (See siman 32 Bach, Eliyahu Rabba and Be'er Heitev), this shiur is only lechatchila according to the Shimusha Rabba.On the other hand, according to the SA and Rama the bayis shel yad must fit on the muscle no higher than the 1/2 way point of the humerus bone. Presumably, a "4x4" bayis can't be accommodated within this space for some/many(?) bar mitzvah boys. If so, lechatchila, shouldn't a smaller bayis shel yad be (temporarily) chosen for such situations rather than to place the bayis higher up and rely on the Gra? (See Mishnah Brurah 27/4; Biur Halachah 32/41 “Ain Lo Shiur” in the name of Shulchan Shlomo; Aruch Hashulchan 27/4, who writes that one who has a sore and can't place the tefillin according to the SA/Rama can rely on the Gra.)