There’s a widespread misconception that special rules apply to long sentences. I’ve known people who believe that any long sentence is a run-on. (Wrong!) Some of my students used to think that any long sentence had to have at least one comma. (Also wrong!) And I’ve known people who always insert a semicolon right in the middle of a long sentence. (Wrong again!)
The truth is that long sentences are just like any other sentences; the usual comma and semicolon rules apply. There’s no special mystery to solve when you write a long sentence.
But there is one issue to watch for: Cramming too much information into a lengthy sentence. You won’t be violating any usage principle, but you might end up with a clumsy sentence that’s difficult to read. Here’s one I came across in a story about author Victor Hugo on a literary website:
On his eightieth birthday in 1881, it took six hours for 700,000 to parade past his house; more than that turned out for his state funeral in 1885, among which would have been a few from the first Romantic Army.
Here’s my version:
On his eightieth birthday in 1881, it took six hours for 700,000 people to parade past his house. More than that turned out for his state funeral in 1885. Among them would have been a few from the first Romantic Army.