Answer: Two fingers-width for both length and width of the ‘bayis’ (and that includes the larger square at the base of the ‘bayis’ – called the ‘titura’!) [Ed: Today, almost all kosher tefillin are considerably larger than the minimum. That is because there must be enough room to fit the parchments upon which the four ‘parshiyos’ (paragraphs from the Torah) are written with halachic precision.]
Question: What is the significance of the two ‘shin’ letters formed on the outside of the ‘bayis shel rosh’? Why does one ‘shin’ have four branches, rather than three?
Answer: The halacha teaches that the two ‘shin’s’ are an absolute requirement of the ‘bayis’, based upon Oral Tradition to Moshe from Sinai. We also know that the letter ‘shin’ represents the first letter of the Divine Name ‘Sha-d-ai’, and that gives the ‘shel rosh’ a higher level of ‘kedusha’ than that of the ‘shel yad’, which does not have the ‘shin’s’. Little is written - at least in the Shulchan Aruch and Mishnah Berura - about the significance of the four-branched ‘shin’. (Ed: I understand that part of it has to do with shape of the “negative” image of a ‘shin’, i.e. if one were to press a three-demensional ‘shin’ into, say a piece of clay, the imprint would have four “branches”! Beyond that, it’s a bit fuzzy…)
Making these raised letters requires great skill on the part of the ‘batim-macher’ (Yiddish for “box-maker”.) They are not separate pieces that are glued onto the sides of the ‘bayis’; they are molded from the leather of the ‘bayis’ itself! Not only that: whenever halacha requires that a letter be raised, it does not permit the formation of that letter to come about through inverse pressure. This means we cannot simply take a “negative” mold and press it into the leather so that a positive shin arises – this would not be valid! What is done in today’s manufacturing of tefillin is that they actually do the negative mold at first, but then they push the raised ‘shin’ back into the leather until it is flush. Then, they pull it out again with tweezers! (Ed: By now, the leather around the ‘shin’ is pliable, because it was once formed and then pushed back in.)
Question: Which ‘shin’ goes on which side of the ‘bayis’, or does it not matter?
Answer: The three-branched ‘shin’ should be on the right of the wearer, and the four-branched ‘shin’ on the left. However, if they were reversed it would still be valid. If both ‘shin’s’ were either three-branched or four-branched, the ‘bayis’ would not be valid!