If it is not a power of two, the nearest power of two less than n is substituted. This sets the size of the squares, measured in pixels. If it is not a power of two, the nearest power of two less than n is substituted. The default ending value is 1. Future versions of POV-Ray will include some system of temporary files or buffers which will eliminate these inefficiencies and limitations.
Mosaic preview is still a very useful feature for test renderings.