Maximum TCP window size

Hi all,

I've got a curious situation here:
A remote HOST system is ftping to one of my machines. The latency
between these two machines is 18ms, which is resulting in a slow FTP
transfer.
So the HOST guys asked me to raise the standard tcp window size from 32
KB (hex 8338), which they can see in their trace, for FTP to 64KB.

Any idea how to do this?

Thanks in advance