This plugin is highly customizable. You can change almost anything without touching its core code.

To remove minlength from message title add following code in your theme’s functions.php (or any custom plugin)

add_filter( 'fep_form_fields', function( $fields ) {
    unset( $fields['message_title']['minlength'] );
    return $fields;
});

To change minlength to 20 use following code
 

add_filter( 'fep_form_fields', function( $fields ) {
    $fields['message_title']['minlength'] = 20;
    return $fields;
});

 
For message area limit, use message_content instead of message_title