What is the minimum width of tefillin straps?
For example: if one's tefillin straps partially tore, how much is required to remain for the tefillin to remain Kosher?
Shulchan Aruch Orach Chaim 27:11 says that the straps should be at least the width of a grain of barley. The Halacha Berura says that this amounts to 1 cm, or 2/5th of an inch, but one should preferably ensure that the Tefillin be at least 1.1 cm, or .44 inches, wide.
The Semag (Aseh #22) rules that the straps should be between a barleycorn and the width of a grain of wheat (I'm not sure how big that is). If they are wider or narrower than that, though, they remain valid.
Rosh and the Tur (OC 27) rule that a barleycorn is the ideal width, but they too accept that narrower straps are kosher.
The Shulchan Aruch (OC 27:11) rules that straps narrower than a barleycorn should still be worn (it seems with a Bracha) when no other straps are available. While the Mishna Berura there limits this to where the straps are at least the width of a grain of wheat, the Aruch haShulchan there (27:22) does not and calls the whole requirement for any minimum width only a Hiddur Mitzva.
Minimally the retzuah must be 9mm wide. For Ashkenazim they should preferably be 11mm according the the Chazon Ish. 10mm width is sufficient for Sephardim.