May young Bar Mitzvah boys wear 4 by 4 arm Tefillin, as customary in Chabad, or are they to purchase smaller hand Tefillin in order so it fit within the first half of the bicep?
The Tefillin Shel Yad needs to be placed on the bicep. However, it is not to be placed above the bicep. This means that it may not be placed beyond the halfway point between the elbow and the shoulder. Furthermore, even within the first half of this space [in which the Tefillin is to be placed], it is not to be placed by its very bottom, below the bicep [which is below the elevated area of the muscle]. [It is to be placed approximately two finger worth [i.e. four centimeters] above the elbow, which is measured from the area where the elbow folds. This applies to even a minority of the Tefillin, that no part of it, including the Titura or Mabarta is to be placed below or above the above-mentioned space.]
Does one measure the half area from the elbow to the top of the shoulder or from the elbow to the armpit? Some write that the area is measured from the area where the elbow bends until the armpit, and the Tefillin may be worn up until half of that area. Accordingly, people with a small built, especially young children of Bar Mitzvah age, must be very careful that no part of the Tefillin rests past this area. Likewise, according to this approach, some question whether children of Bar Mitzvah age may wear 4×4 Tefillin as is customary amongst Chabad Chassidim. [see next Q&A] Others, however, write that the area is measured from the area where the elbow bends until the top of the shoulder from the outside, and the Tefillin may be worn up until half of that area. Accordingly, even people with a small built and children of Bar Mitzvah age can wear 4×4 Tefillin without worry of it passing beyond the halfway point.
If one has a small arm and the Tefillin reaches beyond the valid area, then nevertheless the Tefillin is to be worn regularly at the required distance from the elbow fold, even if the top of the Tefillin extends past the bottom half of the bicep. Accordingly, Bar Mitzvah boys who are accustomed to wear 4 by 4 Chabad Tefillin, may do so even if it extends past the first half of the bicep. This certainly applies according to the understanding brought in the previous Q&A regarding the proper way of measuring the halves, and that it is measured from the crease of the elbow until the shoulder. A proper and accurate measurement of this type shows that even children of Bar Mitzvah age who have a small body frame have plenty of room to wear the 4×4 Tefillin in the correct area.
 Literally: “In the elevated part of the flesh that is by the area of the arm between the elbow and the shoulder, which is called the Kibores.” [Admur ibid; See M”A 27:1; Taz 27:2; Menachos 37a]
The reason: As the verse [Devarim 11:18] states “And you shall place these words [on your heart]” from which we learn that it must be placed opposite the heart. This refers to the bicep which is the area that is exactly opposite the heart. [Admur ibid; Menachos 37b]
 Admur ibid; Rama 27:1; Hagahos Semak 153:1 in name of Shimusha Raba
Other opinions: Some Poskim rule that one may wrap the Tefillin even past the halfway mark, and it is valid to be worn throughout any area of the bicep. [Gr”a brought in M”B 27:4; See Piskeiy Teshuvos 27:1 footnote 6] Accordingly, some Poskim rule that in a time of need one may rely on this opinion and position the Tefillin past the half area. [M”B 27:4 regarding large Tefillin and if there is no room on the arm and 27:29 regarding a case of injury on the arm; Aruch Hashulchan 27:4; Ben Ish Chaiy Chayeh Sarah 6 regarding those who wear two pairs of Tefillin simultaneously; Kaf Hachaim 27:3] Other Poskim, however, completely negate this. [Yabia Omer 1:3; 9:108] According to the lenient approach to wear it past the upper half in a time of need, a blessing may likewise be recited. [Kaf Hachaim ibid; Implication of M”B 27:4 and 27:29] However, other Poskim rule that a blessing may not be said. [Chayeh Adam 14:10; Yabia Omer ibid]
 Admur ibid; M”A 27:2; Bedek Habayis; M”B 27:4 that this invalidation applies according to all
 Sefer Tefila Lemoshe Tefillin 7:7; Piskeiy Teshuvos 27:1; Sefer Oter Yisrael Besifara p. 52
 M”B 27:4 “It is very common by the large Tefillin that the Ketzitza falls beyond the bicep.”; Chofetz Chaim Shaar Hatorah 7; Piskeiy Teshuvos 27:1 based on M”B 27:34 that this applies likewise to the Mabarta being within the space of the upper half
 See Hiskashrus 756 and other volumes for a lengthy discussion on this subject
 Implication of all Poskim who emphasize the armpit in their measurement, including: Michaber 27:1, Rama ibid, Semak 153; Halichos Shlomo 4 footnote 39; Piskeiy Teshuvos 27:1
 Shraga Hameir 6:46; Piskeiy Teshuvos 27:1
 See M”B 27:4 “It is very common by the large Tefillin that the Ketzitza falls beyond the bicep.”
 Implication of Admur 27:2 and Siddur “Between the elbow and shoulder” and so is implied from all Poskim who write the shoulder in the measurement, and not the armpit, including: Rambam Tefillin 4:2; Mordechai Menachos 37a in name of Rabbeinu Tam; Rosh Halachos Ketanos Tefillin 18; Tur 27; Rav Yosef Yitzchak Belinav, printed in Hiskashrus ibid, that so applies according to all, and so is implied from Tosafus Menachos 37a that the intent of the term “armpit” is really shoulder; Sefer Oter Yisrael Besifara p. 52 in article of Rav Yochanon Rubinovitch
 Rav Belinov ibid that based on his measurements of the arms of ten Bar Mitzvah boys, there is 20-21 cm between the crease of the elbow and shoulder, which gives enough space for the 4×4 Tefillin [which takes up a length of about 6-7 cm] to be worn; See also Halichos Shlomo 4 footnote 39 that we do not change the Jewish custom and make the Tefillin smaller in order for those with smaller arms to fit it within the Shiur.
 M”B 27:4; Halichos Shlomo 4:27 that we do not change the Jewish custom and make the Tefillin smaller even though the size of our Tefillin today reaches beyond the Shiur.
 Rav Belinov ibid; Sefer Oter Yisrael Besifara p. 54