When you create a ppt, create a slide and add a textbox, define the font bold 
or italic, the font size will trun to 1. It's too small to seen.
Check the below code:
   public static void test1() {
      try{
         File ppt = new File("D:/temp/a.ppt");
         //HSLFSlideShow hshow = new HSLFSlideShow(new FileInputStream(ppt));
         HSLFSlideShow hshow = new HSLFSlideShow();
         SlideShow show = new SlideShow(hshow);

         Slide slide = show.createSlide();
         
         TextBox box = new TextBox();
//       set box content
//         box.setItalic(true);
         box.setText("Text Box!!");
         // set the box's size
         Rectangle rec = new Rectangle(100,100,100,100);
         box.setAnchor(rec);
         // set text box's color
         //box.setFillColor(Color.BLACK);
         box.setBackgroundColor(Color.BLUE);
         // set text box's font
         box.setFontName("Arial", show);
         box.setFontSize(20);
         box.setBold(true); 
         
         box.setFillColor(Color.BLUE);
         slide.addShape(box);
         
         // line shape
         Line line = new Line();
         line.setAnchor(new java.awt.Rectangle(50, 50, 100, 20));
         line.setLineColor(new Color(0, 128, 0));
         line.setLineStyle(Line.LineDashDotSys);
         slide.addShape(line);
                 
         // write the file
         hshow.write(new FileOutputStream(ppt));
         Trace("Export successful!  " + ppt.getAbsolutePath());
      }catch(Exception e) {
         e.printStackTrace();
      }
   }