Hi sazees kina , in this case you have to get the reference of each column & the set its properties.
e.g :
DATA: lo_column TYPE REF TO cl_salv_column.
TRY.
lo_column = lo_cols->get_column( 'COLNAME' ).
lo_column->set_long_text( 'COLUMNHEADING' ).
lo_column->set_output_length( 20 ).
CATCH cx_salv_not_found.
ENDTRY.
Thanks & Regards-
Vishal