The polling interval is the frequency (in seconds), of polling message QUEUE. This setting helps you manage any probable stress on message QUEUE. Usual recommended setting is 2-5 seconds.

The window size is more relevant for Asynchronous request between ESME and SMSC and defines the max number of allowed number of unacknowledged transactions. It helps to regulate throttling by ESME. 

Window Size should be configured very judiciously depending on various factors such as network latency, IOPS capability of your application. Recommended value is between 30-50 with an environment where network latency is upto 40 ms. It can be fine tuned further. However, excessive higher value of windows size may lead to Throttling Error from SMSC  (error code 0x00000058, ESME has exceeded allowed message limits) or even can trigger Buffer Overflow at ESME end.