Hello Gowtham
The initial window size that is indicated in the SYN portion of the three way handshake is determined based on the size of the receive buffer of the device, the speed of the device, and the speed of the device’s interface to the network. All of these factors are taken into account by the device’s operating system or firmware, in order for the device to specify an initial windows size. This will then change over time, as traffic is evaluated, and as flow control mechanisms kick in.
I hope this has been helpful!
Laz