The choice of paper size depends on the plotter configured for your system.
Each different plotter will have a standard list of available paper sizes. You can change the paper size for a layout by using the CanonicalMediaName property.
You can also specify the units for your layout using the PaperUnits property. This property takes one of three values: acInches, acMillimeters, or acPixels. If your plotter is configured for raster output, you must specify the output size in pixels.