I suggest that you check this to find out more about how you can work with responsive text.
Alternatively, we do have a responsive text feature which you can access from the Cornerstone settings. While you are editing the page with Cornerstone click on the left bottom settings icon and go to Page Settings tab:
There you will have an option to set a specific HTML class to act as a responsive text.
Then you can go back to the element that you added and want to have a responsive text and click on the customize tab and add the class there:
Alternatively, you can use the content area element and use the Responsive Text shortcode to do the same thing.