In the following code, btn3 should have a font size of 32px since it should inherit the 16px font size and double it. The font size ends up being (on Mac) 52px, which is 4 times the default 13px font size.

import javafx.application.Application;
import javafx.beans.value.ChangeListener;
import javafx.beans.value.ObservableValue;
import javafx.scene.Scene;
import javafx.scene.control.*;
import javafx.scene.layout.VBox;
import javafx.scene.text.Font;
import javafx.stage.Stage;

public class Main extends Application {
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        launch(args);
    }

    public void start (final Stage stage) throws Exception {
        
        final Button btn1 = new Button("16px \"system\"");
        btn1.setId("btn1");
        btn1.fontProperty().addListener(new ChangeListener<Font>() {

            public void changed(ObservableValue<? extends Font> ov, Font t, Font t1) {
                System.out.println("btn1 old: " + String.valueOf(t) + ", new: " + String.valueOf(t1));
                btn1.setText(Double.toString(t1.getSize()) + "px \"" + t1.getFamily() + "\"");
            }
        });
        
        final Button btn2 = new Button("32px \"System\"");
        btn2.setId("btn2");
        btn2.setFont(Font.font("system",32));

        final Button btn3 = new Button("-fx-font-size: 2em;");
        btn3.setId("btn3");
        btn3.setStyle("-fx-font-size: 2em;");
        btn3.fontProperty().addListener(new ChangeListener<Font>() {

            public void changed(ObservableValue<? extends Font> ov, Font t, Font t1) {
                System.out.println("btn3 old: " + String.valueOf(t) + ", new: " + String.valueOf(t1));
                btn3.setText(Double.toString(t1.getSize()) + "px \"" + t1.getFamily() + "\"");
            }
        });
        

        VBox vbox = new VBox();
        vbox.setStyle("-fx-font: 16px \"system\";");
        vbox.setSpacing(5);
        
        vbox.getChildren().addAll(btn1,btn2,btn3);
        
        Scene scene = new Scene(vbox, 800, 400);
        stage.setScene(scene);
        stage.show();
        
    }
}