GreG is a completely device independent graphic system. This means that a sequence of plot actions will give exactly the same drawing on any device, except possibly for a global scale factor (contrary to many systems where the plot is distorted by some X/Y aspect ratio). It is then possible to define a unique Physical Coordinate unit. By convention in GreG this Physical Unit corresponds to 1 true centimeter on a paper output, if you use the /EXACT option in the metacode device driver. The Plot Page is defined in terms of such units.
A default Plot Page is defined at GreG initialization. It is an A4 format page (30 by 21 cm) with the LANDSCAPE orientation, i.e. with the largest dimension along the X coordinate. Plot pages of other dimensions can be defined using command
SET PLOT_PAGE SizeX SizeY
Another frequently used Plot Page corresponds to the A4 format with the PORTRAIT orientation (long dimension corresponding to Y) which can be accessed by command SET PLOT_PAGE PORTRAIT.