The Matrix Themes come with custom js plugin "scaled text".
The js automatically scales the text if it has more than 50px and adapts its size to it's parent div.
It works only for module "TEXT" when you add custom font size, so using the span tag.
See below example:
This is large and scaled text that automatically adapts to its parent div
If you resize the browser window, you can see how the first example scales the text. It doesn't work for the second example because it's module "Heading".
The headings h1, h2, h3 can be setup in Design> Font Settings and the recommended size are:
For more details, please check the documentation of Matrix Themes.