You can find the following about this in the ABAP documentation:
The LOW and HIGH fields of a selection option are displayed in a length up to 18 bytes long (scrollable up to 45 bytes). If you define a length longer than 45, fields are truncated on the selection screen after the 45th character. This affects the first line of the SELECT-OPTIONS table. You can, however, use SUBMIT to pass longer selection options to a report if they are specified with the addition NO-DISPLAY and thus do not appear on the selection screen. Without NO-DISPLAY, the fields are then truncated whenever the selection screen is processed in the background ( SUBMIT without VIA SELECTION-SCREEN).