You can't select the actual "Screen" by clicking on it in the designer. Even if you have no other elements added. If you add any element, you get the typical resize behavior you would expect, but since you cannot select the screen, you cannot resize it or even view the properties.
You are able to select the screen in the "Objects and Timeline" list. Once you do, you get the expected resize handles and can even view the properties using the tab on the right and resize it manually there.