How To Set Size For Panel With Graphics2D Object

I am writing a program to solve Sudoku puzzles. I create the user interface using AWT/Swing.One panel of the main window (JFrame object) contains a Graphics2D component only (a grid for the sudoku puzzle). The problem is that this panel is too small for its contents when it is created. I have tried many things to make it bigger (like the setSize method of the panel), but to no avail. Who can tell me how I can make the panel with a single Graphics2D object larger?

Below is the code for the main window, which is a JFrame object. I set its size to 400 x 400 to have ample room for all panels. I then create two panels: a sudoku panel that contains the sudoku grid and a button panel that contains two buttons. The button panel has a flow layout.

I then create the main panel. This has a vertical BoxLayout. Finally I add the sudoku panel and the button panel to the main panel and the main panel to the frame.

public class ViewController_Graphics extends JFrame {
JButton solveButton = new JButton("Solve");
JButton nakedSinglesButton = new JButton("Naked Singles");

public ViewController_Graphics(SudokuModel model) {
super("Sudoku");
setSize(400,400);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setLookAndFeel();
// create sudoku panel

[code]....

When I run the program, the following Window shows up:the button panel is exactly in the middle (and it remains in the middle, no matter how I resize the window). And there is not enough room for the sudoku panel. How can I make enough room for the sudoku panel when the window is created?