Since the host MCU has a limited memory space to receive large data, the RX_PACKET_POOL_SIZE defines the Rx buffer pool size. The TCP_WND is used to control rx data flow which depends on it.
The RX_PACKET_POOL_SIZE could be defined in the makefile as below:
GLOBAL_DEFINES += RX_PACKET_POOL_SIZE=10
For sample code you may take a look at .../apps/test/console application and .../apps/test/console/console.mk make file for definitions.