That's what Teasers are for — the Teaser is the shortened version of the content, presented in RSS feeds (among other places, such as the site's front page).
When editing a node in Drupal 6, use the "Split summary at cursor" button to split the node's body into a Summary/"Teaser" and "Full view".
Then go to your site's /admin/content/rss-publishing page, and set "Feed content" to "Title plus teaser".
Additionally, on your site's /admin/content/node-settings page, you can configure the default length to use to split nodes.
For some additional customization, try the Read More Link module (but be aware of this issue).