It’s important not to use <h1> tags (and other headings) just to affect font size. You should use CSS to target whatever HTML element is already there.
It’s also important not to edit a theme directly, because when you update that theme, all of your changes will be lost. To avoid that problem, see if your theme has a “custom CSS” area where you can put in your styles. If not, there are plugins that will allow you to add your own CSS outside of the theme, or you could create a child theme.
Typically, for the best user experience, you’ll want to use media queries to make the font sizes responsive. You can use media queries within custom CSS areas, plugins, or child themes.