With regard to writing sentences, Mark Twain said: "At times he [the author] may indulge himself with a long one, but he will make sure there are no folds in it, no vaguenesses, no parenthetical interruptions in its view as a whole; when he has done with it, it won't be a sea-serpent with half its arches under the water; it will be a torchlight procession."
    As soon as I read that I felt an urge to check my sentences. Have I indulged in parenthetical loops? Do I use lots of subordinate clauses? Twain's sentence demonstrates that providing long sentences are punctuated properly and do not wander off into asides they are perfectly understandable.
    Variety in sentence length is desirable. It keeps prose lively. Shorter sentences are generally held to increase the tempo. The more tense the action, the shorter and more direct the sentences. As with all rules, this one has its exceptions. In Samuel Beckett's story, 'Ping', the longer the sentences, the more the tension/action intensifies. But then, Beckett's story breaks many other rules too. I recommend reading it. You can find it online at http://www.samuel-beckett.net/ping.html. This story also demonstrates that short sentences can be every bit as ambiguous as long ones. Enjoy!