Changing between these different units affects the document field as described below. There are several different fixed document sizes. If units are set to inches, the American A, B, C , D and E sizes are available. If units are set to millimeters, the International A4, A3, A2, A1 and A0 sizes are available. If units are set to pixels, no fixed document sizes are available.
Any size is available for Portrait or Landscape .
Any size can be set in the width and height boxes as long as they are between the minimum of 32 x 32 pixels, and the maximum document size supported by your scanner. This determines the number of Dots per Inch, which is the scanners true resolution. Note that higher resolutions than those supported by the currently configured scanner are grayed. For example, the 800DPI option is grayed for Vidar TruScan 600 but enabled for Rank Xerox 7336 and Vidar Flash. Each one has it's own scanner related default parameters.