How long are the Tefilin Shel Rosh straps supposed to be? What is the minimum Shiur and what is the maximum?
The bar mitzva volume of Nit'ei Gavri'el #52 quotes the following practices:
- Some say the straps should reach the legs. (Mishna B'rura 27:35)
- Some say the right strap should reach below the belt and the left to the navel. (Shulchan Aruch 27:11)
- Some say the right strap should reach the navel and the left to the chest. (Tur 27)
The latter two sound like neither maxima nor minima, but exact preferable ranges.
According to the Rambam
The length of the straps of the head tefillin should be sufficient to surround the head, tie the knot, and extend on either side of the head until they reach the navel or slightly above it.