Chapter 15 Unified Threat Management for FortiOS 5.0 : Custom Application & IPS Signatures : Custom signature keywords : TCP header keywords : window_size
  
window_size
Syntax: --window_size [!]<window_int>;
Description:
Check for the specified TCP window size. You can specify the window size as a hexadecimal or decimal integer. A hexadecimal value must be preceded by 0x. To have the FortiGate search for the absence of the specified window size, add an exclamation mark (!) before the window size.