Change font-size

Hello community.
So I have created this option where user can change his own font , color of buttons . it works just fine . But , since some fonts are bigger than others , the letters appear smaller . how to zoom up all font sizes at the same time , I can’t use font-size because this will change all sizes .
ps : I am customizing the desk , not the website .

Hello @bahaou how did you add this option?