Here's something I just discovered. If you let Drupal generate a teaser automatically for you, it normally takes the first 600 characters of your story. If you want to decide the teaser length for yourself, put:
<!--break--> (exactly as written) in your body text where you want the teaser to end. You can't make the teaser greater than 600 characters but you can choose a tidy point for it to finish.
Not the most exciting thing in the world, I admit, but somebody might find it useful!
