Writing a long sentence is easy; doing it well is hard. Amateur writers think falsely of what a long sentence is, how it works, and when to use it. Short sentences are good, and you should use them often. An idea portrayed quickly carries more force than the same idea winded into a long, gasping journey. But prose composed of four- to ten-letter sentences is dull. You fall asleep. That's where the long sentence enters.
I cannot define a length that qualifies a long sentence. Twenty words may seem a lot to you, or forty, or a hundred. You know that this is a short sentence. But you also understand, assuming you have enough experience with the English language, that this sentence, grammatically correct, is a long sentence (and it has twenty-seven words). The danger with long sentences is that they can conquer the writer—the amateur loses control of structure, grammar, voice, or the central idea. Some fiction writers think stream of consciousness implies long sentences. Bad writers have heard that Jame Joyce wrote a twenty-something page sentence (not grammatically correct), and therefore all they have to do is to write their thoughts without any period. That's not how it works. Joyce perfectly understood the language he used and was an accomplished author before he attempted the windingly long sentence. Bad attempts at long sentences sound like James Joyce without his beauty or control. Here is a bad long sentence:
For the few who write fiction, allow me some tips that will improve your prose. If people have told you that your fiction stinks, follow these rules and write with good grammar. Even if your story is shit, your prose will be clean.
Vague words: Very, actually, just, really, quite, all speak loud but say little. I have written about these words before, but I'll repeat it again: use concrete descriptions. If I say The hall was very long, how is that different from saying The hall was long? Vague qualifiers attempt detail, but fail to add anything other than pulp, fattening fiction as greasy food fattens the body.
Proper quotations: "Hey, babe." The man said. If you append dialogue with he said, she said, or any variant thereof, it must be part of the same sentence; the dialogue must end with a comma, exclamation point, or question mark. In proper form: "Hey, babe," the man said or "What's up?" she said. Do not use a comma if you do not follow with a speaker attribution: "Hey, babe," the man drove alongside the girl.
Immediate dialogue: Keep dialogue real, curt, and honest. Avoid words such as so and now because they, like vague words, only fatten the useless meat of conversation. "So how are you?" becomes "How are you?" in cleaner writing.