public class HelloTextField extends Application {
    @Override
    public void start(Stage stage) {
        final TextField a = new TextField();
        a.setPromptText("Enter text here");
        a.setId("a");

        final TextField d = new TextField();
        d.setId("d");
        d.setOnAction(new EventHandler<ActionEvent>() {
            @Override public void handle(ActionEvent event) {
                a.setStyle("-fx-font-size: " + Integer.valueOf(d.getText()) + "pt");
            }
        });

        VBox box = new VBox(5);
        box.setFillWidth(false);
        box.getChildren().addAll(a, d);
        stage.setScene(new Scene(box, 500, 500));
        stage.show();
    }

    public static void main(String[] args) {
        launch(args);
    }
}