To reproduce run following code
1 test case: choose some item in choice box, press "reset" button. Choice box become shoter.
2 test case: try to add some long item "looooooooooo00000ong" . Choice box doesn't change the size.


import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.ChoiceBox;
import javafx.scene.control.TextField;
import javafx.scene.layout.HBox;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class ChoiceBoxSa extends Application {
    ChoiceBox cb;

    protected Scene getScene() {
        cb = new ChoiceBox();

        HBox content = new HBox(5);
        content.getChildren().add(cb);
        reset();

        HBox addBox = new HBox();
        final TextField toAdd = new TextField();
        Button addBtn = new Button("Add");
        addBtn.setOnAction(new EventHandler<ActionEvent>() {

            public void handle(ActionEvent t) {
                cb.getItems().add(toAdd.getText());
            }
        });


        HBox clearBox = new HBox();
        Button resetBtn = new Button("Reset");
        resetBtn.setOnAction(new EventHandler<ActionEvent>() {

            public void handle(ActionEvent t) {
                reset();
            }
        });
        clearBox.getChildren().add(resetBtn);

        addBox.getChildren().add(toAdd);
        addBox.getChildren().add(addBtn);
        VBox controls = new VBox();
        controls.getChildren().add(clearBox);
        controls.getChildren().add(addBox);
        content.getChildren().add(controls);
        Scene res = new Scene(content);
        return res;
    }

    protected void reset() {
        cb.getItems().clear();
        cb.getItems().add("1234567890");
        cb.getItems().add(3);
        cb.getItems().add(false);
    }

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

    @Override
    public void start(Stage stage) throws Exception {
        stage.setScene(getScene());
        stage.show();
    }
}