Example of a JavaFX DirectoryChooser widget that can resize to fit the screen.

FolderSelector.java

package com.desive.stages.components;

import com.desive.utilities.StageDictionary;
import javafx.geometry.Insets;
import javafx.scene.Group;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.control.TextField;
import javafx.scene.layout.ColumnConstraints;
import javafx.scene.layout.GridPane;
import javafx.stage.DirectoryChooser;
import javafx.stage.Stage;

import javax.swing.filechooser.FileSystemView;
import java.io.File;

/*
 Created by Jack DeSive on 12/31/2017 at 6:30 PM
*/
public class FolderSelector extends GridPane {

    private TextField directoryPathField;
    private Button selectDirectoryButton;
    private Label groupLabel;
    private Group buttonGroup = new Group();

    private DirectoryChooser directoryChooser = new DirectoryChooser();
    private File selectedDirectory = null;

    private Stage windowOwner;
    private double gridHGap = 10D, gridPadding = 10D;

    public FolderSelector(String groupLabel, StageDictionary stageDictionary, Stage windowOwner) {

        this.windowOwner = windowOwner;

        setupDirectoryChooser(stageDictionary);
        setupComponents(groupLabel, stageDictionary);

        this.setHgap(gridHGap);
        this.setPadding(new Insets(gridPadding));
        this.add(this.groupLabel, 0, 0, 2, 1);
        this.add(directoryPathField, 0, 1);
        this.buttonGroup.getChildren().add(selectDirectoryButton);
        this.add(buttonGroup, 1, 1);
    }

    public File getSelectedDirectory() {
        if(selectedDirectory != null && selectedDirectory.isDirectory()){
            return selectedDirectory;
        }
        return null;
    }

    public void resize() {
        this.setPrefWidth(windowOwner.getWidth());
        setupColumnConstraints();
    }

    private void setupColumnConstraints(){
        buttonGroup.applyCss();
        buttonGroup.layout();
        ColumnConstraints column1 = new ColumnConstraints();
        column1.setPrefWidth(this.getPrefWidth()-buttonGroup.getLayoutBounds().getWidth());
        this.getColumnConstraints().clear();
        this.getColumnConstraints().add(column1);
    }

    private void setupDirectoryChooser(StageDictionary stageDictionary) {
        directoryChooser.setTitle(stageDictionary.SELECT_DIRECTORY_POPUP_TITLE);
        directoryChooser.setInitialDirectory(FileSystemView.getFileSystemView().getHomeDirectory());
    }

    private void setupComponents(String groupLabel, StageDictionary stageDictionary) {
        directoryPathField = new TextField();
        selectDirectoryButton = new Button(stageDictionary.COM_DIRECTORY_SELECT_BUTTON_TEXT);
        this.groupLabel = new Label(groupLabel);
        selectDirectoryButton.setOnAction(event -> {
            selectedDirectory = directoryChooser.showDialog(windowOwner);
            if(selectedDirectory != null){
                directoryPathField.setText(selectedDirectory.getAbsolutePath());
            }
        });

    }

}

Main.java

public class Main {
    
    public void start(Stage primaryStage) throws Exception {
        
        // StageDictionary is just my class for strings. (Can be replaced)
        FolderSelector folderSelector = new FolderSelector("Select a Directory", new StageDictionary(), primaryStage);
        GridPane grid = new GridPane();
        grid.getChildren().add(folderSelector, 0, 0);
        
        Scene scene = new Scene(grid, 700, 800);
        primaryStage.setScene(scene);
      
        // Here is the magic. Add listeners so FolderSelector.class knows when to resize.
        scene.widthProperty().addListener(event -> folderSelector.resize());
        scene.heightProperty().addListener(event -> folderSelector.resize());
      
        primaryStage.show();
        folderSelector.resize(); // Capture first layout (Must be after Stage.show())
    }
  
    public static void main(String[] args) {
        launch(args);
    }
  
}