That's strange, on my end fullscreen mode does not stretch the screen at all and the sides of the screen simply have black bars, while testing on two different windows machines I was not able to reproduce the stretching. Could you let me know what kind of monitor/display you're using (including its resolution, if you know it), and what graphics card you have? Also, does the game display correctly in windowed mode and only become stretched when you switch to fullscreen?
Also, you can return to the title screen in-game! Press P to pause and click on Return to Hub if in a level, then press P to pause in the hub world and click Return to Menu to get back to the title screen.