Where on one's arm should the tefillin be?
The tefillin shel yad should be located on the muscular part of the arm, the bulge which rises between the elbow and the shoulder. The tefillin should be placed on the down slope of the muscle that leans towards the elbow. Some poskim allow the entire bulge of the muscle. Tefillin that are too large to fit on the slope may be placed on any part of the bulge. It is preferable to put them higher on the arm rather than lower, because the area of the arm below the muscle towards the elbow is not acceptable. It is also important to make sure that the tefillin are not pushed down to this area when pulling a sleeve over them.
(סימן כז, סעיף א, ס"ק ג-ד, וביה"ל ד"ה בבשר וד"ה בראש; ביאורים ומוספים דרשו, 3)