Sentences should be simple but varied.  Use a combination of sentence structures to keep your reader’s attention.  There is no formula for determining sentence length, but shorter sentences keep the reader’s attention.  Generally, short sentences are also easier to understand.  A reader loses attention when a sentence is more than 20 words in length.  You do not necessarily have to count all the words in your sentences or try to limit them, simply be aware that the readability is increased when sentences are short.