Setting Font Size for Tooltip is not applicable.

Compile and run this sample:

package tooltiptest;

import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Tooltip;
import javafx.scene.layout.StackPane;
import javafx.scene.text.Font;
import javafx.stage.Stage;

/**
 *
 * @author Ladislav Török local
 */
public class TooltipTest extends Application {

    @Override
    public void start(Stage primaryStage) {
        String javaVersion = System.getProperty("java.version");
        String javafxRuntimeVersion = System.getProperty("javafx.runtime.version");
        System.out.println("Java version: " + javaVersion);
        System.out.println("JavaFX runtime version: " + javafxRuntimeVersion);
        
        Tooltip btnTooltip = new Tooltip();
        btnTooltip.setFont(new Font(70));
        btnTooltip.setText("Hello, World!");
        
        Button btn = new Button();
        btn.setText("Say 'Hello World'");
        btn.setTooltip(btnTooltip);
        btn.setOnAction((ActionEvent event) -> {
            System.out.println("Hello, World!");
        });

        StackPane root = new StackPane();
        root.getChildren().add(btn);

        Scene scene = new Scene(root, 300, 250);

        primaryStage.setTitle("Hello World!");
        primaryStage.setScene(scene);
        primaryStage.show();
    }

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

}

See screenshots:
javafx 8u40b22-ea - bad: http://i60.tinypic.com/30k52yw.png
javafx 8u0b132 - good: http://i57.tinypic.com/2qdxyde.png (tooltip is trimmed, but it is ok, because screenshot is for application window)

Note: when I using JavaFX 8.0 b132 - all is ok., other JavaFX platforms I don't have installed.