By default, the length of an input field on the selection screen is the same as the length of the parameter in the ABAP program. However, you can define the visible length of a parameter as smaller than its actual length (as is also possible for input/output fields on screens):
PARAMETERS <p> ... VISIBLE LENGTH <len> ...
If <len> is smaller than the field length of <p>, the input field is displayed in the length <len>. In all other cases, the parameter is displayed in its full length. Part of the contents of parameters with a shorter visible length will be obscured, but the field is automatically set to scrollable.
PARAMETERS: p1(10) TYPE c VISIBLE LENGTH 1,
p2(10) TYPE c VISIBLE LENGTH 5,
p3(10) TYPE c VISIBLE LENGTH 10.
WRITE: / 'P1:', p1,
/ 'P2:', p2,
/ 'P3:', p3.
The three parameters P1, P2, and P3 have the same length (10), but are displayed in different lengths on the selection screen. However, you can enter up to ten characters in any of the fields.