How to change font size with HTML in Java Swing JEditorPane?

Use HTMLEditorKitt to change the font size with HTML. With that, use the JEditorPane setText() method to set HTML:

HTMLEditorKit kit = new HTMLEditorKit();
editorPane.setEditorKit(kit);
editorPane.setSize(size);
editorPane.setOpaque(true);
editorPane.setText("<b><font face=\"Verdana\" size=\"30\"> This is a demo text with a different font!</font></b>");

The following is an example to change font size with HTML in Java Swing JEditorPane:

Example

import java.awt.BorderLayout;
import java.awt.Container;
import java.awt.Dimension;
import javax.swing.JEditorPane;
import javax.swing.JFrame;
import javax.swing.text.html.HTMLEditorKit;
public class SwingDemo extends JFrame {
   public static void main(String[] args) {
      SwingDemo s = new SwingDemo();
      s.setSize(600, 300);
      Container container = s.getContentPane();
      s.demo(container, container.getSize());
      s.setVisible(true);
   }
   public void demo(Container container, Dimension size) {
      JEditorPane editorPane = new JEditorPane();
      editorPane.setEditable(false);
      HTMLEditorKit kit = new HTMLEditorKit();
      editorPane.setEditorKit(kit);
      editorPane.setSize(size);
      editorPane.setOpaque(true);
      editorPane.setText("<b><font face=\"Verdana\" size=\"30\"> This is a demo text with a different font!</font></b>");
      container.add(editorPane, BorderLayout.CENTER);
   }
}

Output