The following should get round the problem (a bit sloppy I'm afraid)

Code:
declare
  cursor c1 is select table_name from dba_tables where owner='TROPP' ;
  tname varchar2(250):= null ;
begin
  for i in c1
    loop
      if length(tname) > 200 then
        dbms_output.put_line(tname);
        tname := null;
      end if;
      tname := tname||i.table_name||',';
    end loop;
  dbms_output.put_line(tname);
end;