code: def size(self) -> Optional[int]:
          
          if not self._parts:
              return 0
  
          total = 0
          for part, encoding, te_encoding in self._parts:
              if encoding or te_encoding or part.size is None:
                  return None
  
              total += int(
                  2 + len(self._boundary) + 2 +  # b'--'+self._boundary+b'\r\n'
                  part.size + len(part._binary_headers) +
                  2  # b'\r\n'
              )
  
          total += 2 + len(self._boundary) + 4  # b'--'+self._boundary+b'--\r\n'
          return total
docs: Size of the payload.