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If I correctly remember the window size defines the dimension of the neighbourhood (around each point) used in the corner detection/refinement process. For example, with (4, 4) you are using a small cell of 9x9 pixels centered in the corner points that you want to refine. The size must be big enough to have enough gradient information to properly find the ideal corner location (if the window is small and the corner is a bit blurred or noisy, the refinement will be propably worse that the original result) but at the same time the window should be small enough to guarantee only one corner inside it. Notice that, the images acquired for the calibration process should guarantee sharp and big chessboards so that you can use a bigger windows size.