Hitchcock's Rule

Hitchcock's rule is that the size of any object in your frame should be proportional to its importance to the story at that moment. 

It is important to know your story to be able to identify what is the most important for a particular shot. 

Typically a close up shot should be used when the detail is most important to the story at that moment. 

Medium shots should be used when it is necessary to show the relationship between two objects. 

The use of wide shots shows the setting of the story or the environment around the subject and how it relates to the story. 

Everything in the frame should be important to the story. You must figure out how important the object is and what you are trying to say with the shot in order to figure out which shot to use.