Big string in SQLPlus I need to write a Select query in bind-variable and this query has more than 2500 symbols. 

Variable sql_txt clob;

Exec :sql_txt := 'Select .....';

But SQL Plus support only &lt;2499 symbols. 
How to fix it?
That limitation is for a single line.

You can break your statement into multiple lines to overcome that limitation, such as:

begin
  :sql_txt := 'select ... ' ||
  ' ... ' ||
  ...
  ' ... ';
end;
/