Limiting the WordPress feed to only a certain limit of characters

You can select between Full text and Summary in the Reading settings:

If you select the Summary, then

a) To control the number of words in the feed summary you can use:

add_filter('excerpt_length','custom_excerpt_length'); 
function custom_excerpt_length( $num_words ){
    return 30; // number of words to show
}

The default number of words in the summary is 55.

b) If you want to use <!--more--> in the post content to define the feed summary, you can use the following:

add_filter( 'the_content', 'custom_content_feed' );
function custom_content_feed( $content ){
    if( is_feed() ){
        // <!--more--> used in the post content: 
        if( strpos( $content, '<span id="more-') !== FALSE ){
            // remove the excerpt length limit
            add_filter( 'excerpt_length', 'custom_long_excerpt_length' ); 
            // get the content before <!--more-->
            $content = stristr( $content, '<span id="more-', TRUE );
            // add the default 'read more' symbols at the end:
            $excerpt_more = apply_filters( 'excerpt_more', ' ' . '[&hellip;]' );
            $content .= $excerpt_more;
         }
    }
    return $content;
}
function custom_long_excerpt_length( $num_words ){
    return 99999;
}

c) You can also use a) and b) together.