Desktop and windows enlarged on MacBook

If you zoomed the whole screen, so that, for example, the menu bar may no longer fit inside the screen: hold down the Control key and do a scroll down gesture (typically dragging with two fingers on the trackpad).

If this is it, and it is something that you do not want to happen, turn it off in System Settings->Accessibility->Zoom

You probably enabled the handicap services in System Settings/Preferences.

command-option-minus key will shrink the magnification until you get the right size usually.