Striving for clarity is essential no matter the length of the email, report, post or note. The length should be enough to get the point across but not too long to loose the reader. For me, the length of a correspondence is usually dictated by the amount of clutter the prose has in it. Striving for reducing the clutter will make the length what it needs to be. Brevity incarnate. David Silverman has a must read post on this. Brief and to the point.
