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I just looked down at the little plastic cover on my arm-tefilin and it had the number 4 on it. I assume that's a size ... while tefilin must be square, there are definitely a variety of sizes out there. Are there now some standards? Does anyone know what they are? E.g. "size 4 is 1.5 inches" or something?

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Saw this too when shopping for replacement cases. –  Yosef Dec 2 '10 at 16:18
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Mine says 35, and by measuring it, that's in millimeters (the bayis, not the titura). So yours is probably 4 cm = 40 millimeters.

(I've heard this size called "daled al daled." I used to think that meant four etzbaos, but that's definitely not the case - an etzba is 20 mm according to R' A.C. Naeh, and about 26 mm according to the Chazon Ish, so tefillin 4x4 etzbaos would be huge.)

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Yeah the dalet al dalet must be a nickname; 4 x 4 centimeters. –  Shalom Dec 20 '10 at 20:02
    
@Shalom. It is centimeters. The Titurim of Tefillin must be more than an Etzba. The Gaonim said the Batim should ideally be 2 Etzba'im. [When the Rabbis say Etzba without qualifying, they refer to a thumb] (Shulchan Aruch Harav 32:63 - hebrewbooks.org/pdfpager.aspx?req=25072&pgnum=156). R' Avraham Chaim Na'eh holds that a thumb is 2cm. hebrewbooks.org/pdfpager.aspx?req=20888&pgnum=326 . Therefore 2 Etzbas is 4cm. –  Menachem Aug 15 '11 at 18:05
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The numbers 34 or 4 found on the batim cases refer to the millimeter measurement of the upper cube of the batim. hence 34mm or 4, referring to 34mm.

To follow the opinions of the Rishonim that hold the batim should be 2 'etzbaos (thumb widths) many will be more strict than the Shulchan Aruch, who says that there is no shiur to tefillin. Most hold that the part of the batim that has to be 2 thumb widths is the titura, the base of the tefillin (not including the maavarta, passageway. Rav Avraham Chaim Naeh holds that 2 thumb widths is 4cm and the Chazon ish holds this is a minimum of 4.8cm and preferably 5cm. However, Chabad holds that the part that has to be 2 thumb widths is the actual bayis, upper cube. They also follow the opinion of Rav Avraham Chaim Naeh. Hence, the upper cube needs to be 4cm x 4cm.

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Can you give some examples of those Rishonim who hold 2x2 'etzbaos is a minimum? –  Double AA Sep 19 '12 at 19:12
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This site has an explanation and a program to help you figure out sizes.

While This site gives a basic outline of Tefillin in general and sizes according to whichever custom you wish to follow.

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