data: lt_fieldcat type lvc_t_fcat with header line.

  CALL FUNCTION 'LVC_FIELDCATALOG_MERGE'
    EXPORTING
      I_STRUCTURE_NAME             = p_structure
    CHANGING
      CT_FIELDCAT                  = lt_fieldcat[].

  data: l_length type i.

  loop at lt_fieldcat.

    l_length = strlen( lt_fieldcat-SCRTEXT_L ).
   
    if l_length > lt_fieldcat-outputlen
    and l_length > lt_fieldcat-dd_outlen.
      lt_fieldcat-outputlen = l_length.
      modify lt_fieldcat transporting outputlen.
    endif.

  endloop.