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Defines a square in screen space, with sides of the specified value in pixels. For example, 128 defines a square of 128 x 128 pixels. The region's bounding box must be larger than this square (and smaller than the maxLodPixels square) in order for the Region to be active.
More details are available in the Working with Regions chapter of the Developer's Guide, as well as the Google Earth Outreach documentation's Avoiding Overload with Regions tutorial.