Beaver Responsive Fonts is a plugin which adds a panel to the Theme customizer which lets you select a minimum and then a maximum font size and then the remaining headings are all scaled based on a perfect ratio value.

https://github.com/jatacid/beaver-responsive-fonts/archive/master.zip

Uses !important declarations and may override other styling that you have. Best to use on a new site so you can control the correct styling easier.