How can i set image section size in opticalFlowLK method(computer vision toolbox)?

I am trying find optical flow vector using lucas kanade method using matlab inbuilt function opticalflowlk, in this method we need to divide each image into smaller section of given window size. By using inbuilt opticalflowlk method i am not able to find how can i set window size for that. If it is not possible then what is the default window size is used by opticalflowlk? I have read all documentation but didn't find the answer anybody having a idea please help