Hi Ankesh,
In our ALV at max you can display only 128 char.for your requirement you have to use text Warping functionality which I don't like personally because of it's look. I face same issues just few day before
so I suggest put a button or simply a icon with hotspot feature on click display the long text by using
FM 'TERM_CONTROL_EDIT'.
Reference link
ABAP Custom Text Editor Pop UP - Code Gallery - SCN Wiki
for use of FM.
Hope this Help You to solve your issues