The paper size of DesignMerge PostScript output is determined by the Paper Size setting that you select on the InDesign Print dialog, which is displayed before the DesignMerge PostScript session begins. Select a specific paper size. Do not select Defined by Driver.

If you have selected an appropriate paper size and the PostScript output is still not printing at the correct paper size, your selected PPD's default paper size setting may be causing a conflict. Review your selected PPD's default settings to confirm the default paper size is appropriate, or select a different PPD for the PostScript output. 

Also, confirm that the paper size property on the print controller or processing application has been set correctly for this job.

For more information about using the DesignMerge PostScript Driver, please see DesignMerge Documentation.