The Zohar writes that the right strap that descends from the knot of the Tefillin Shel Rosh should be slightly longer than the left strap. Both straps must reach the stomach, and the right strap should be slightly longer.

The Shulhan Aruch rules that all Tefillin straps must have a minimum width of a barley seed. In contemporary terms, this is measured as .4 inch, or 1 cm. Some Poskim maintain that one should ensure the straps are at least .44 inch, or 1.1 cm, wide. Generally speaking, when one purchases Tefillin the straps are wider than the minimum required width. With time, however, the part which is tied around the upper tends to stretch, and its width shrinks. One must therefore ensure that the width of the entire strip is no less than .4 inch, and preferably no less than .44 inch.