A tweet can have a maximum of 140 characters. A guide on how to use Twitter, well, that's another matter. The UK government has one, and it took its author, Neil Williams, 20 pages and 36,215 characters to create it.
The guide, embedded below, is actually quite an interesting read. Besides giving a clear cut explanation of what Twitter is and how (and why) the UK government should go about using it, it enumerates a number of third party tools (for example, bit.ly is to be used for links because it offers traffic analysis), talks about risks of using Twitter (for example, publishing embargoed news too early), and discusses the value of retweeting. Unlike some other Twitter guides, like the one from Wall Street Journal, it's not focused on forbidding things and telling employees what NOT to do, which is a good example of how these types of guides should be done.
While we're at on the subject of very long Twitter guides, you might want to check Mashable's Twitter Guide Book, a collection of resources which helps you understand what Twitter is and how to make the best of it.