I wanted to use that quote, but I couldn't recall the attribution or exact wording and my google-fu wasn't up to finding it. Thanks!
Well, via Wikiquote the exact wording is:
Je n'ai fait celle-ci plus longue que parce que je n'ai pas eu le loisir de la faire plus courte.
...which probably didn't help your efforts, because translations vary and are often reworded slightly to suit the tastes of the quoter.
But I recalled that it was about a letter, and googling on letter shorter longer was sufficient to find the attribution. Perhaps my mind was already well-primed for finding relevant quotes and this illustrates well why I feel almost palpably stupider without internet access.
See this great little rationalist video here.