To remove minlength and maxlength from both message title and message content add following code in your theme’s (child theme’s if any) functions.php

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