Dimensions Zone
The dimensions and resolution for the image to be rendered are provided in the Dimensions zone.
The Format drop-down list provides access to the most common sizes and formats. Some, such as User Defined or User Defined in Units allow the dimensions to be provided directly as values. By default, the format Defined by view is proposed.
The 
button next to the Resolution box is a shortcut to standard resolutions expressed in dpi (dots per inch). If the resolution is provided in units other than dpi, the chosen standard resolution will be converted to the selected units.
In the Orientation zone, select one of the radio buttons for Portrait or Landscape. This choice is not available if you have selected one of the predefined formats: Defined by Viewport, Defined by Product, or Fill Viewport.