Paragraph sizes can help enhance the mood/atmosphere of the story.
For example, long paragraphs (not too long) should be used to describe thing in detail, and to keep the story moving at a slow pace.
Whereas short paragraphs should be used to speed up the pace, and when the atmosphere is tense.
One sentence paragraphs can change the flow of writing, and should be used to create a shock or to remind the reader something.