https://www.elegantthemes.com/blog/divi-resources/the-complete-guide-for-creating-fluid-typography-in-divi-6-methods 

#1 Using a VW Length Unit for Font Size

This is an H1 Title

Your content goes here. Edit or remove this text inline or in the module Content settings. You can also style every aspect of this content in the module Design settings and even apply custom CSS to this text in the module Advanced settings.

#2 Using Calc() and VW Length Unit for Font Size

This is an H1 Title

This is H2

H1 is 3em

H2 is 2em

Body is 1em

font-size: calc(1em + 0.5vw) !important

#6 Using clamp() to Create Fluid Typography in Divi

This is an H1 Title

This is H2

H1 is 3em

H2 is 2em

Body is 1em

font-size: clamp(16px, 4vw, 26px) !important;