You can select in the Project Settings what happens when you resize the window (Project Settings->Display->Window->Stretch). You can either hide parts (disabled), shrink/expand resizing all objects (2d), or shrink/expand resizing the rendered image (viewport, which will pixelize it like resizing a normal image). And in Aspect you can select which one to keep correct: height or width. Height usually works best for me, so in widescreen monitors you get extra space in the sides instead of hiding top and bottom parts