Changing the length of a selection screen field

Hi,

i have a doubt and describing it below.please can anyone help me with regard to this.

in my program i have a field PSPID as a selection screen field.it's defined using select options.as you all know PSPID is of 24 char length.now i want to change the selection screen field length to 5char with out changing in the table PROJ.

we have a syntax SELECT-OPTIONS : FIELD FOR _____ VISIBLE LENGTH 5.

using this syntax we can change the visibility to 5.but we can enter upto 24.

my requirement is i should restrict the user to 5 char only.please can anyone help me?

Reagrds,

Chaitanya.