Short sentences are usually best.
Newspapers teach journalists to write a single thought in a sentence. That way the meaning is clearer.
Full stops. Use plenty. They keep sentences short. This helps the reader.
Much as I love short sentences, too many in a row makes writing boring and hard to read. They can also be uneconomical.
As Harold Evans points out in Newsman’s English:
Often it is wasteful to introduce a subject and predicate for each idea. The subordinate clause in a complex sentence can state relations more precisely and more economically than can a string of simple sentences or compound sentences joined by and, but, so, etc.
There’s another reason to use complex sentences in your writing. They add rhythm.
Use too many short sentences and your copy will have a staccato flow annoying and distracting readers. Use too many long sentences and your writing will lack pace. You may lull your readers to sleep.
A similar logic applies to paragraphs. View them as bundles of closely related thoughts.
There’s no hard and fast rule about the best length for paragraphs. It’s a good idea to minimise the number of one sentence paragraphs you write. As with sentences, vary the pace. Too many consecutive short paragraphs is annoying. Too many long ones is hard work for the reader. Both approaches are difficult to read.
Above all else use paragraphs to make your writing easier to read.