Here is a useful quote from Zinsser's "On writing well", a timeless guide to all such matters:

Keep your paragraphs short. Writing is visual—it catches the eye before it has a chance to catch the brain. Short paragraphs put air around what you write and make it look inviting, whereas a long chunk of type can discourage a reader from even starting to read.

But don't go berserk. A succession of tiny paragraphs is as annoying as a paragraph that's too long. I'm thinking of all those midget paragraphs—verbless wonders—written by modern journalists trying to make their articles quick 'n' easy. Actually they make the reader's job harder by chopping up a natural train of thought.