java.awt.print How to set default page size to A6

I have a java class with a static method that prints a JPanel which implements a custom interface (called MyPrintablePanel).

The problem is, when the print dialog appears (Page setup dialog box), the default paper size is "Letter". I want the default paper size to be A6 so that I don't have to manually choose A6 each time I print.

Here is the code:

package designer.office.utils;

import designer.office.printables.MyPrintablePanel;
import java.awt.Component;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.print.PageFormat;
import java.awt.print.Paper;
import java.awt.print.Printable;
import java.awt.print.PrinterException;
import java.awt.print.PrinterJob;
import javax.print.attribute.Size2DSyntax;
import javax.print.attribute.standard.MediaSize;
import javax.print.attribute.standard.MediaSizeName;
import javax.swing.JPanel;

/**
 * A class to print a JPanel
 */
public class MyPrinter implements Printable {
    
    final MyPrintablePanel comp;

    public MyPrinter(MyPrintablePanel comp){
        this.comp = comp;
    }
    
    public static void printJPanel(MyPrintablePanel jPanel){

        PrinterJob pjob = PrinterJob.getPrinterJob();
        MediaSize isoA6Size = MediaSize.getMediaSizeForName(MediaSizeName.ISO_A6);
        float[] size = isoA6Size.getSize(Size2DSyntax.INCH);
        Paper paper = new Paper();
        paper.setSize(size[0] * 72.0, size[1] * 72.0);
        paper.setImageableArea(0.0, 0.0, size[0] * 72.0, size[1] * 72.0);
        PageFormat preformat = new PageFormat();
        preformat.setPaper(paper);
        
        PageFormat postformat = pjob.pageDialog(preformat);
        //If user does not hit cancel then print.
        if (preformat != postformat) {
            //Set print component
            pjob.setPrintable(new MyPrinter(jPanel), postformat);
            if (pjob.printDialog()) {
                try {
                    pjob.print();
                } catch (PrinterException ex) {
                    System.out.println("Failed to print: " + ex.getMessage());
                }
            }
        }
    }

    @Override
    public int print(Graphics g, PageFormat format, int page_index) 
            throws PrinterException {
        if (page_index > this.comp.getNumOfPages() - 1) {
            return Printable.NO_SUCH_PAGE;
        }

        // get the bounds of the component
        Dimension dim = comp.getSize();
        double cHeight = dim.getHeight()/this.comp.getNumOfPages();
        double cWidth = dim.getWidth();

        // get the bounds of the printable area
        double pHeight = format.getImageableHeight();
        double pWidth = format.getImageableWidth();

        double pXStart = format.getImageableX();
        double pYStart = format.getImageableY();

        double xRatio = pWidth / cWidth;
        double yRatio = pHeight / cHeight;


        Graphics2D g2 = (Graphics2D) g;
        g2.translate(pXStart, pYStart - page_index * pHeight);
        g2.scale(xRatio, yRatio);
        comp.paint(g2);

        return Printable.PAGE_EXISTS;
    }
}

I'm printing (or rather my client is printing) from a HP Laser Jet Professional P1102 printer with A6 paper in its tray.