You might have accidentally zoomed in or out of the scree. If you have google chrome as your browser, you can fix this by clicking on the magnifying glass on the upper right side and setting it to 100%. With the game open, that is. If you don't have google chrome, you must first click on the purple (or blueish, I can't tell the color) boarder of the emulator. That step will pause your game. Next, hold down the CTRL button and press 0 (zero) on the number pad and it will return to regular size. Make sure you don't let go of the CTRL button when you press 0 (zero)! After screen returns to normal size is when you can let it go.