Hi Amine,
It is typo.. it should be 255
READ TABLE lt_objtxt INTO lrec_soli INDEX l_tab_lines.
lrec_doc_chng-doc_size = ( l_tab_lines - 1 ) * 255 + STRLEN( lrec_soli ).
Say table has 10 lines and the 10th line has only 5 characters.
So, you multiply 9* 255 and then add only 5 of the last line (since it was not complete line).
Please let me know if you need further clarification.
Cheers,
Raju