It's usually translated as "I would have written a shorter letter, but I did not have the time." The more literal translation is "I made this [letter] very long, because I did not have the leisure to make it shorter."
I smile because the less literal translation shows he could've made the sentence shorter if he'd had more time.
This observation was prompted by a bit in Amazon's New Plan to Pay Authors Every Time Someone Turns a Page - The Atlantic:
If I work hard to be pithy and crisp in order to keep the reader’s skittish attention, there will be fewer pages to read, and less money to be earned. Writing concisely is an art that takes a lot of time and careful editing. As Blaise Pascal said...Payment by the word has always bothered me because it rewards sloppy writing. Payment by the page isn't better. In the world I want, writers will be able to write without having to pit artistic needs against commercial ones.
ETA: "The secret of being a bore is to tell everything." —Voltaire