I spent some time looking for BOX_WIDTH because I needed to change the width of my admin boxes. It’s where you’d expect it but it isn’t so obvious…
Public viewable boxes…
/includes/application_top.php
~line 61
Admin boxes…
/admin/includes/application_top.php
~line 52
BTW, your lines might be different depending on your version of osCommerce.
Happy Coding,
Thuan