I have just performed this test: The image below has a 10x10 pixel green square in the middle, surrounded by a 20x20 blue square. When the WB grid is set to 5x5 and centered on the image it shows, of course, only 25 green pixels. When the grid loupe is enlarged to its maximum, 17x17 (289 pixels) and centered it shows 100 green pixels and 189 blue pixels. However, in both cases the WB set is absolutely the same, which demonstrates that the WB is calculated only from the central 25 pixels no matter the grid size.
Thanks everyone for the info. I learned something. Guess we're stuck with one sample size, 5x5 unless we do multiple samples and average the results.