Hello,
Unfortunately all input fields needs to be escaped (sanitized), so html tags are not allowed. This is rule of thumb here on WP org https://codex.wordpress.org/Validating_Sanitizing_and_Escaping_User_Data .
It might be possible to define width for the subtitle for particular slider or just one slide with some custom css code, so if you want to try to solve this like that please share a link to a page where Rolo Slider is present and explain what needs to be done.
Best Regards!
