You should start with
https://docs.mendix.com/howto/guis/create-a-custom-theme-with-the-mendix-ui-framework
when you have your styling folders, there is a file
/styles/sass/custom/_custom-variables.scss
you should shange the line with
// Default Font Size & Color
$font-base-size: 12px;
This might also be helping
https://ux.mendix.com/
Keep in mind, that depending on the theme you are using, there might be some fixed sizes on certain elements. You will have to adjust these manually.