- Max of 65536 

The WSize or Window Size is used to determine how much data can be sent without acknowledging that it has been received.  Smaller window sizes mean the proxy can receive less data before sending back an ACK packet.  Larger window sizes mean more data can be received before sending back an ACK.  The number is the amount of data in bytes.

This can be used to fingerprint proxies and is recommended to be changed to fit your OS / Browser emulation.  This is also used in conjunction with / modified by the "WScale" TCP option!