Q. One wants to replace an old pair of Tefillin (that was used for over 20 years) with a new pair.
The new pair is smaller than the old one, but is more Mehudar (parshiyos, batim and retzhuos). Does the size matter; does the new pair have to be same as, or bigger than the old pair?
What about RaSH”I and R”T? Do both pairs have to be of same size, or can one be bigger than the other?

Thank you

A. Or Zarua (Tefilin 568) quotes Shimusho Rabbo that the size of Tefillin should be two fingerbreaths (etzabaos) by two fingerbreaths, but if they are more or less, they still are kosher. Biur Halacha (27: 9) maintains that the Tefillin should not be bigger than four etzbaos by four etzbaos including the titura (base) and the ma’avarta (the extension where the retzua goes through). Horav Chaim’s Noe zt”l (Shiurei Torah) opinion is that an etzba measures 2.0 cm. While the Chazon Ish measurement is 2.4 cm.
The smallest tefillin are sized such that two pairs can be worn simultaneously; both those made according to the opinion of Rashi in added to those made according to the opinion of Rabbeinu Tam. Many Jews of Sephardic descent wear both pair of tefillin at the same time. When donned together some also maintain different sizes for this two pairs. The standard size tefillin made for such use today is about 2.0 to 2.2 cm. (Yalkut Yosef 34: 2 – he maintains that this includes the titura and ma’avarta, which is surprisingly small.
See also Minhagei Hachida p. 18). Ase Lecha Rav (p. 137) writes that each pair is 4.0 cm and the correct place for tefillin is a total of 8.cm. The largest size recommended for tefillin are as large as two of the smallest pair tefillin combined. This size of tefillin is commonly worn by Chabad Chasidim ranges from 40 to 43 mm.
As far as the tefillin being considered mehudar (beautiful or enhanced), a greater emphasis should be placed on the quality of the parshios. The size of the batim, as long as it is not too small or too large, as mentioned above, does not really matter.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a.