iftcpwnd <ifname> <tcp_wnd>

[root@sylixos:/root]# help iftcpwnd       
Set/Get TCP Window size for net interface
eg. iftcpwnd lo0        (get 'lo0' TCP Window size setting)
    iftcpwnd en1 65535  (set 'en1' TCP Window size to 65535)
iftcpwnd [ifname] [tcp_wnd]
[root@sylixos:/root]# 

[root@sylixos:/root]# iftcpwnd en1
TCP Window size: 65535
[root@sylixos:/root]# iftcpwnd en1 65534
TCP Window size: 65534
[root@sylixos:/root]#