What’s the default TCP window size of iperf3

Part of Vista and Server 2008 is a next-generation TCP stack... it's auto-tuned, so there is no more TCPWindowSize tweak.

As far as changing it, you can turn auto-tuning on or off with these commands...

netsh int tcp set global autotuninglevel=normal
netsh int tcp set global autotuninglevel=disable

The TCP window size is how much data can be "in flight" on the network. The TCP receive buffer is how much data can be buffered on the recipient's end.

Normally, a TCP stack will not allow data to be sent if it has no room for it in its receive buffer. Otherwise, if the data is received before the receiving application consumes some of the data in the buffer, the data would have to be throw away by the receiving TCP stack.

But the receive buffer can be much larger than the window.

With the settings you've shown (14,600 / 87,380), this end will allow the other end to send 14,600 bytes. As it receives data, it will update the window to allow to other end to send the lesser of 14,600 bytes or 87,380 bytes less the number of bytes waiting in its receive buffer.