import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.ChoiceBox;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class ChoiceBoxDefaultSize extends Application {

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

    @Override
    public void start(Stage stage) throws Exception {
        VBox vb = new VBox();
        
        ChoiceBox cb = new ChoiceBox();
        
        vb.getChildren().add(cb);
        
        Scene scene = new Scene(vb, 300, 200);
        stage.setScene(scene);
        stage.show();
    }
}