One of the most frequently-given bits of advice in regards to prose is to vary up the length of your sentences. If every sentence in a paragraph is the same length or the same format, your writing will lack the natural rhythm and flow that most readers unconsciously pick up on in good prose. A paragraph of short sentences will feel choppy; a paragraph of long sentences can become fatiguing. A mix of the two is usually preferable.
The problem is that long sentences are more difficult to write well than shorter sentences. It’s like the difference between building a small cottage and building a towering skyscraper: both skyscrapers and longer sentences require more complex and robust construction. The longer the sentence, the greater the chance that some sort of grammatical error will creep in and ruin things.
But even when a long sentence is grammatically correct, it has a greater chance of confusing readers. The primary purpose of sentences and paragraphs is to divide information into logical, bite-size chunks that can be quickly analyzed and assimilated. Readers are trained to absorb information one sentence at a time, and a long sentence forces them to process more information at once. For example:
The house was little more than a dingy shack crammed between two other huts at the westernmost edge of what passed for a village on this hard, narrow path that wound through the wilted woods off of the main highway.
That’s a lot of information to throw at your readers in one chunk. By the time a reader gets to the end of that sentence, they barely remember where the sentence started. A sentence should usually contain from one to three pieces of information (usually closely-related information). Let’s look at the information given in our example sentence:
The house was little more than a dingy shack (Description of the house)
crammed between two other huts (Physical situation of the house)
at the westernmost edge of (Location of the houses)
what passed for a village (Editorial commentary on size of village)
on this hard, narrow path (Relative location of village)
that wound through the wilted woods (Location of path)
off of the main highway. (Relative location of woods)
That’s seven pieces of information—too much for one sentence, especially one without a colon or semicolon. We should break the sentence into more than one. How do we decide where to break it up? Look at the bits of information; we have three pertaining to the house, two pertaining to the village, and two more that are really just intended to give us more details about the location of the village. So we’ll delete the unneeded bit about the path, which isn’t actually serving the intended aim of describing the location of the village, and then we’ll divide this sentence into two—one about the house, and one about the village:
The house was little more than a dingy shack crammed between two other huts. They stood at the westernmost edge of what passed for a village in the wilted woods off the main highway.
Now, we could also end that first sentence with a semicolon (or maybe even an em-dash) if, for some reason, we were determined to keep all of that information in one sentence. That’s usually the difference between long sentences that work and the ones that don’t—those that work make careful use of breaking punctuation to create the needed pauses and divides in the information they contain, thereby making it easier for readers to assimilate one chunk at a time.