The preferable width is 11mm according to the Chazon Ish and 10mm according to Rav Chaim Naeh. The minimal width for making a Brachah is 9mm. If a Retzuah has been squeezed together at one spot making it less than the minimal width but it can be pulled back in to the minimal width it is acceptable . If however it has been stretched out and is narrower than the minimal width at any spot it must be replaced.
The end of the Retzuos may be cut on an angle even though they will be less than the minimal width.
Lesson: Do not tie the straps on very tight or wrap them on the box very tightly as this will lead to eventual stretching and invalidating the Tefillin straps