Customizing RSS length

[Nebthtet]

Is there a way to define the length of what's exported in RSS feed from a Pages Database? I don't want whole articles to be there because a certain forum sicks our contents and reposts it in their internal part thus lowering out pageviews. For the time being I've been forced to turn off RSS for databases but it's like fighting rats with a Kalashnikov  - too much collateral damage.

Something like "read more" feature from Wordpress would be ideal. How can I achieve that?