Fixer la hauteur des JTextField

for Component c : frame.getContentPane().getComponents() {
   if(c instanceof JTextField){
      //Là, tu peux faire ce que tu veux sur le JTextField en effectuant un cast au besoin
   }
}

import java.awt.Component;
import java.awt.Container;
import java.awt.Rectangle;
import java.awt.Window;

import javax.swing.JFrame;
import javax.swing.JTextField;

/**
 * @author Jonathan Guéhenneux
 */
public class FieldVisitor {

	private static final int PREFERRED_FIELD_HEIGHT = 20;

	public static void main(String[] args) {

		JFrame frame = new JFrame();
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		frame.setLayout(null);
		frame.setSize(200, 120);
		JTextField field = new JTextField();
		field.setBounds(10, 10, 100, 50);
		frame.add(field);
		frame.setVisible(true);

		FieldVisitor fieldVisitor = new FieldVisitor();
		fieldVisitor.visitAllWindows();
	}

	/**
	 * 
	 */
	public void visitAllWindows() {

		Window[] windows = Window.getWindows();

		for (Window window : windows) {
			visit(window);
		}
	}

	/**
	 * @param container
	 */
	public void visit(Container container) {

		Component[] components = container.getComponents();

		if (container instanceof JTextField) {
			visit((JTextField) container);
		}

		for (Component component : components) {

			if (component instanceof Container) {
				visit((Container) component);
			}
		}
	}

	/**
	 * @param textField
	 */
	public void visit(JTextField textField) {

		Rectangle bounds = textField.getBounds();
		bounds.height = PREFERRED_FIELD_HEIGHT;
		textField.setBounds(bounds);
	}
}