Your internet browser and/or Flash Player may be zoomed too far out or in. Check the View menu at the top of the screen. Click the Zoom In or Zoom Out selection to change how much of the game is displayed in the window. You can also use the Actual Size selection to return the zoom to its default level.
Alternately, you can use CTRL-0 (Zero) on your PC or Command-0 on your Mac to set the zoom to default.