setVisibleRowCount does not work if called before filling of items list. The resulted height of drop-down length is much less then it should be.

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.ComboBox;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class Main extends Application {

    @Override
    public void start(Stage stage) {
        VBox content = new VBox();
        final ComboBox cb = new ComboBox();
        cb.setEditable(true);
        stage.setScene(new Scene(content, 400, 400));
        Button btn = new Button("Reset");
        btn.setOnAction(new EventHandler<ActionEvent>() {
            public void handle(ActionEvent t) {
                cb.setVisibleRowCount(4);
                for (int i = 0; i < 10; i++) {
                    cb.getItems().addAll(String.valueOf(i));
                }
            }
        });
        content.getChildren().addAll(cb, btn);
        stage.show();
    }

    public static void main(String[] args) {
        launch(Main.class, args);
    }
}