To choose a new board size, choose Settings > Board > Size..., move the slider to the desired value, and click OK.
Use the recommended value to create a board that occupies most of your screen. This value represents the length, in pixels, of the height and width of the actual game board, not including the window, menu, and status message.
If you're setting this option from the command line, you can use any value between 200 and 2000. If you're setting this option from the dialog box, you can use any value between 200 and 2000, selected in increments of 40.