Write a paragraph of narrative, 100-150 words, in sentences of seven or fewer words. No sentence fragments! Each must have a subject and a verb.
Write a half page to a page of narrative, up to 350 words, which is all one sentence.
short sentences don't have to consist of short words; long sentences don't have to consist of long ones. If you find this happening in your prose, you might try, just out of contrariness or curiosity, to counteract the tendency.