Strange but simple question.
I own a pair of Tefillin and because of the width of my arms I always struggle to get the final strap wrappings for the arm Tefillin down to my finger knuckles. I can accomplish it but it requires an extremely tight grip and multiple attempts. It makes it hard for me to easily do the mitzvah like other Jews.
Is it legally acceptable to have your strap length increased or is the length a religious standard which no Jew has the liberty of altering?
Are there shops which do this?