Here is an updated version of the code toxi posted to define din A0, A1, A2, A3, A4 paper formats using iText. Might be helpful for someone.
             
             

              

             
             

              
import processing.pdf.*;
              
import com.lowagie.text.PageSize;
              
import com.lowagie.text.Rectangle;
              


              
void setup() {
              
 // create window @ A3 landscape using OPENGL
              
 pageSize(com.lowagie.text.PageSize.A3,true,OPENGL);
              
 // create window @ US Letter size, portrait using default renderer
              
 //pageSize(com.lowagie.text.PageSize.LETTER,false,JAVA2D);
              
}
              


              
/**
              
* Convenience method to be used instead of the normal size() command.
              
* Creates window matched to a given paper size
              
*
              
* @param r a predefined constant of the iText PageSize class
              
* @param isLandscape true, if you want landscape orientation
              
* @param renderer class name of the Processing renderer to use
              
*/
              
void pageSize(com.lowagie.text.Rectangle r, boolean isLandscape, String renderer) {
              
 if (isLandscape) {
              
 size((int)r.getHeight(),(int)r.getWidth(),renderer);
              
 } else {
              
 size((int)r.getWidth(),(int)r.getHeight(),renderer);
              
 }
              
}