How can I define the RSS feed content length?

I usualy make a RSS feed in my template. Just copy the RSS feed you have in the wp-include/feed-rss2.php
Step two is registering it instead of the original:

// override feeds
function own_rss( $for_comments ) {
    //it is in my theme/feed/rss2.php
    $rss_template = get_stylesheet_directory() . '/feed/rss2.php';
    if( file_exists( $rss_template ) ) {
        load_template( $rss_template );
    } else {
        do_feed_rss2( $for_comments ); // Call default function
    }
}
remove_all_actions( 'do_feed_rss2' );
add_action( 'do_feed_rss2', 'own_rss', 10, 1 );

Step three is adjusting the your own RSS to your needs.

also look at this:
http://codex.wordpress.org/Customizing_Feeds