The pixel size is the span of one detector pixel, in micrometers. (The pixels are assumed to be square, which seems to be true for all currently marketed detectors). The detector width is the width or height of the entire pixel array, in centimeters (if the array is non-square, the larger of the two dimensions is chosen). This is the quantity that is referred to as the "diameter" in Batch mode. The detector diagonal is the diagonal span of the pixel array, equal to sqrt(2) times the detector width, and often quoted by detector manufacturers as the "diameter". 2-theta max is the arctangent of the sample-detector distance divided by the radius (half the detector width). The Q-range and 2-theta max are automatically updated every time the pixel width, detector width or detector diagonal are changed.