Fixed height of the_content()

If you run a wp_kses() on the_excerpt() (through a filter) you can whitelist specific HTML tags and such. In your case, the code would probably look something like this:

if( is_page( 'your_page' ) ) {
    add_filter( 'the_excerpt', 'wpse44363_filter_the_excerpt' );
}
function wpse44363_filter_the_excerpt( $excerpt ) {
    // this allows no html, you can allow only links or something of that nature
    $allowed_html = array();
    return wp_kses( $excerpt, $allowed_html );
}