You can select a predefined size or a user defined size. When you select the user defined size you must specify its X and Y dimensions.
The predefined output size Original is equal to the original plotter paper size (as specified in the Paper Size dialog). If you use this size for exporting a non-trimmed image, you ensure the 1:1 scale for that export.
For all sizes you can specify an offset from the left margin (on the X axis) and from the bottom margin (on the Y axis).